Feature request checklist

• There are no issues that match the desired change
• The change is large enough it can't be addressed with a simple Pull Request
• If this is a bug, please file a Bug Report.

Change
There is currently no way in the TypeProvider interface to discover the fields available in a message.

Example

for _, field := range provider.MessageFields(msgName) {
        fieldType, found := provider.FindFieldType(msgName, field)
        ...do something with field...
}

Alternatives considered
Could use proto reflection if you have an instance about, but won't work for a compiler server where foreign proto definitions are imported by FileDescriptorSet.