botocore: trace presigned URL generation

[rite7sh]

Fixes #4040

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Added a new unit test test_presigned_url_is_traced
• Verified that a span named botocore.presigned_url is emitted when
  botocore.signers.RequestSigner.generate_presigned_url is invoked
  (e.g. via rds.generate_db_auth_token)

To reproduce:

pytest instrumentation/opentelemetry-instrumentation-botocore/tests/test_botocore_instrumentation.py -k presigned

[rite7sh]

Hey @xrmx , this PR implements tracing for RequestSigner.generate_presigned_url (incl. RDS IAM auth) as discussed in #4040.
Would love your review when you have a moment, thanks :)