[Python] Set a Beam user agent on the boto3 S3 client

[goanpeca]

Addresses #39573

apache_beam.io.aws.clients.s3.boto3_client.Client builds its boto3 client without a botocore Config, so every S3 request Beam makes goes out with the stock boto3 user agent and carries nothing identifying the caller as Beam. The other cloud clients in the Python SDK all tag themselves already: gcsio.py sends apache-beam/<version> (GPN:Beam), bigquery_tools.py sends apache-beam-<version>, and the Datastore helper sends beam-python-sdk/<version>. The S3 client is the odd one out.

This passes Config(user_agent_extra='apache-beam/%s' % beam_version.__version__) when constructing the client. botocore appends user_agent_extra to the end of the composed user agent, so the boto3, botocore, platform, and language segments are all preserved and only a suffix is added. The version comes from apache_beam.version.__version__, the same source gcsio.py uses. The practical benefit is that Beam's S3 traffic becomes identifiable in server access logs, which helps when diagnosing throttling or endpoint problems against Amazon S3 and against any other S3-compatible endpoint reached through --s3_endpoint_url.

Tests: a new ClientUserAgentTest in client_test.py constructs the client from a default S3Options() and asserts the composed client.client.meta.config.user_agent contains apache-beam/<version>. It is guarded with @unittest.skipIf(boto3_client is None, ...) following the pattern already used in s3filesystem_test.py, and client construction needs no credentials and no region, so it runs offline. yapf and ruff are clean on both files.

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