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ARROW-10675 [C++][Python] Support AWS S3 Web identity credentials

Added basic support for explicitly specifying use of web identity credentials (it's already part of the default credentials provider chain) in the C++ API. Will add Python API in a separate PR.
Also refactored some existing code to make serialization and deserialization in Python more structured by adding an enum class `S3CredentialsKind` which uniquely identifies the credential type being used.
Also adds some missing error-checking and tests in the Python API.

Closes #10088 from sahil1105/sahil/s3-web-id-creds

Authored-by: sjgupta2 <sjgupta2@illinois.edu>
Signed-off-by: Antoine Pitrou <antoine@python.org>