### What changes were proposed in this pull request?

Added a new `dropFields` method to the `Column` class.
This method should allow users to drop a `StructField` in a `StructType` column (with similar semantics to the `drop` method on `Dataset`).

### Why are the changes needed?

Often Spark users have to work with deeply nested data e.g. to fix a data quality issue with an existing `StructField`. To do this with the existing Spark APIs, users have to rebuild the entire struct column.

For example, let's say you have the following deeply nested data structure which has a data quality issue (`5` is missing):
```
import org.apache.spark.sql._
import org.apache.spark.sql.functions._
import org.apache.spark.sql.types._

val data = spark.createDataFrame(sc.parallelize(
      Seq(Row(Row(Row(1, 2, 3), Row(Row(4, null, 6), Row(7, 8, 9), Row(10, 11, 12)), Row(13, 14, 15))))),
      StructType(Seq(
        StructField("a", StructType(Seq(
          StructField("a", StructType(Seq(
            StructField("a", IntegerType),
            StructField("b", IntegerType),
            StructField("c", IntegerType)))),
          StructField("b", StructType(Seq(
            StructField("a", StructType(Seq(
              StructField("a", IntegerType),
              StructField("b", IntegerType),
              StructField("c", IntegerType)))),
            StructField("b", StructType(Seq(
              StructField("a", IntegerType),
              StructField("b", IntegerType),
              StructField("c", IntegerType)))),
            StructField("c", StructType(Seq(
              StructField("a", IntegerType),
              StructField("b", IntegerType),
              StructField("c", IntegerType))))
          ))),
          StructField("c", StructType(Seq(
            StructField("a", IntegerType),
            StructField("b", IntegerType),
            StructField("c", IntegerType))))
        )))))).cache

data.show(false)
+---------------------------------+
|a                                |
+---------------------------------+
|[[1, 2, 3], [[4,, 6], [7, 8, 9]]]|
+---------------------------------+
```
Currently, to drop the missing value users would have to do something like this:
```
val result = data.withColumn("a",
  struct(
    $"a.a",
    struct(
      struct(
        $"a.b.a.a",
        $"a.b.a.c"
      ).as("a"),
      $"a.b.b",
      $"a.b.c"
    ).as("b"),
    $"a.c"
  ))

result.show(false)
+---------------------------------------------------------------+
|a                                                              |
+---------------------------------------------------------------+
|[[1, 2, 3], [[4, 6], [7, 8, 9], [10, 11, 12]], [13, 14, 15]]|
+---------------------------------------------------------------+
```
As you can see above, with the existing methods users must call the `struct` function and list all fields, including fields they don't want to change. This is not ideal as:
>this leads to complex, fragile code that cannot survive schema evolution.
[SPARK-16483](https://issues.apache.org/jira/browse/SPARK-16483)

In contrast, with the method added in this PR, a user could simply do something like this to get the same result:
```
val result = data.withColumn("a", 'a.dropFields("b.a.b"))
result.show(false)
+---------------------------------------------------------------+
|a                                                              |
+---------------------------------------------------------------+
|[[1, 2, 3], [[4, 6], [7, 8, 9], [10, 11, 12]], [13, 14, 15]]|
+---------------------------------------------------------------+

```

This is the second of maybe 3 methods that could be added to the `Column` class to make it easier to manipulate nested data.
Other methods under discussion in [SPARK-22231](https://issues.apache.org/jira/browse/SPARK-22231) include `withFieldRenamed`.
However, this should be added in a separate PR.

### Does this PR introduce _any_ user-facing change?

Only one minor change. If the user submits the following query:
```
df.withColumn("a", $"a".withField(null, null))
```
instead of throwing:
```
java.lang.IllegalArgumentException: requirement failed: fieldName cannot be null
```
it will now throw:
```
java.lang.IllegalArgumentException: requirement failed: col cannot be null
```
I don't believe its should be an issue to change this because:
- neither message is incorrect
- Spark 3.1.0 has yet to be released

but please feel free to correct me if I am wrong.

### How was this patch tested?

New unit tests were added. Jenkins must pass them.

### Related JIRAs:
More discussion on this topic can be found here:
- https://issues.apache.org/jira/browse/SPARK-22231
- https://issues.apache.org/jira/browse/SPARK-16483

Closes #29322 from fqaiser94/SPARK-32511.

Lead-authored-by: [email protected] <[email protected]>
Co-authored-by: fqaiser94 <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>