feat: add bpd.options.bigquery.requests_transport_adapters option

[tswast]

This allows for overriding requests-based settings such as the maximum connection pool size.

Todos:

• Wait for fix: prevent creating unnecessary client objects in multithreaded environments #1757 first
• Add unit tests that request_transport_adapters can be used to set the max pool size.
• Add a code sample for how to set the max pool size.

Fixes internal issue b/419106112 🦕

[tswast]

Wait for #1757 first. Done!

[tswast]

test_credentials_need_reauthentication is a real failure. We no longer pass in a credentials object.

[tswast]

test_credentials_need_reauthentication failure is a real one. No _credentials anymore. Investigating how best to address now.