[v2] Multi-auth trait support

[aemous]

Issue #, if available:

• Multi-auth trait support boto/botocore#3233
• Sigv4a signing region set config bugfix boto/botocore#3279

Description of changes:

• Ported the linked botocore PRs above to add multi-auth trait support

Description of tests:

• Passed the unit/functional tests provided with the original commit.
• Manually tested specifying multiple auth types in an overridden (ECS) service model. Logs are shown below to verify this:
• Manually tested specifying the environment variable AWS_SIGV4A_SIGNING_REGION_SET and config variable sigv4a_signing_region_set and verified the X-Amz-Region-Set headers were properly updated in requests.

"auth":["aws.auth#sigv4a", "aws.auth#sigv4", "smithy.api#httpBearerAuth"]

<AWSPreparedRequest stream_output=False, method=POST, url=https://ecs.us-west-2.amazonaws.com/, headers={'X-Amz-Target': b'AmazonEC2ContainerServiceV20141113.CreateCluster', 'Content-Type': b'application/x-amz-json-1.1', 'User-Agent': b'aws-cli/2.17.57 md/awscrt#0.21.2 ua/2.0 os/macos#23.6.0 md/arch#arm64 lang/python#3.12.4 md/pyimpl#CPython cfg/retry-mode#standard md/installer#source md/prompt#off md/command#ecs.create-cluster', 'host': b'ecs.us-west-2.amazonaws.com', 'X-Amz-Security-Token': b'REDACTED', 'X-Amz-Date': b'20240925T153938Z', 'X-Amz-Region-Set': b'us-west-2', 'Authorization': b'AWS4-ECDSA-P256-SHA256 Credential=REDACTED, SignedHeaders=content-type;host;x-amz-date;x-amz-region-set;x-amz-security-token;x-amz-target, Signature=3044022063bdd92d8b01f913ac2f26c1f924701a0b45067ff0a36e647d11cdd23def86b8022029c22a6b7b762d1caf095c25043c9a579322a983acc488ae04abf7fcfaa34b07', 'Content-Length': '2'}>

"auth":["aws.auth#sigv4", "smithy.api#httpBearerAuth"]

<AWSPreparedRequest stream_output=False, method=POST, url=https://ecs.us-west-2.amazonaws.com/, headers={'X-Amz-Target': b'AmazonEC2ContainerServiceV20141113.CreateCluster', 'Content-Type': b'application/x-amz-json-1.1', 'User-Agent': b'aws-cli/2.17.57 md/awscrt#0.21.2 ua/2.0 os/macos#23.6.0 md/arch#arm64 lang/python#3.12.4 md/pyimpl#CPython cfg/retry-mode#standard md/installer#source md/prompt#off md/command#ecs.create-cluster', 'X-Amz-Date': b'20240925T153823Z', 'X-Amz-Security-Token': b'REDACTED', 'Authorization': b'AWS4-HMAC-SHA256 Credential=REDACTED, SignedHeaders=content-type;host;x-amz-date;x-amz-security-token;x-amz-target, Signature=3ef073a161442c4f35a6c0d952ba8f823cd5cc4dd2a3c90a1aa804a99223714e', 'Content-Length': '2'}>

"auth":["smithy.api#httpBearerAuth"]

File "/awscli/botocore/auth.py", line 845, in add_auth
    raise NoAuthTokenError()
botocore.exceptions.NoAuthTokenError: Unable to locate authorization token

(This error verifies that the port works for singleton list of bearer since it requested the token in the first place)

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[kdaily]

Looks good! We should clean up some commit history before merging, but otherwise its 🚢