aws.go ParseSigV4 fails to parse the Authorization unless extra spaces are added

[superbet-t2]

Expected behavior:
The aws.go ParseSigV4 method should handle the Authorization header being a single continuous string as mentioned here: https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_sigv-authentication-methods.html
Examples of authorization headers not having spaces in are here: https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html
As an example
ParseSigV4("AWS4-HMAC-SHA256 Credential=AKIAIOSFODNN7EXAMPLE/20130524/us-east-1/s3/aws4_request,SignedHeaders=host;x-amz-content-sha256;x-amz-date,Signature=fea454ca298b7da1c68078a5d1bdbfbbe0d65c699e0f91ac7a200a0136783543")
Should succeed and not return an error.

Current behavior:
The current logic requires a space after every every comma. So the above Header in that format works:
ParseSigV4("AWS4-HMAC-SHA256 Credential=AKIAIOSFODNN7EXAMPLE/20130524/us-east-1/s3/aws4_request, SignedHeaders=host;x-amz-content-sha256;x-amz-date, Signature=fea454ca298b7da1c68078a5d1bdbfbbe0d65c699e0f91ac7a200a0136783543")

Bug details:

• Teleport version: Latest
• Recreation steps: My personal use case comes from using aws-beam library for erlang but any 'technically correct' implementation of aws apis will run into issues.