Set client session User-Agent when fetching server metadata and JWKs

[alex-ball]

What kind of change does this PR introduce?

This is a fix for issue #704.

Requests for server metadata and JWKs (from the /.well-known/ Open ID Connect endpoints) as part of authorize_redirect() can fail if the user agent string is missing or inappropriate. This fix adds the Authlib user agent string in exactly the same way as OAuth1Base._get_oauth_client() and OAuth2Base._get_oauth_client().

Tests have been added to demonstrate the fix in both the synchronous case (Flask client) and asynchronous case (Starlette client). Since they test headers sent during load_server_metadata, the two test_oauth2_fetch_metadata tests could be adapted or extended to test PR #710 (dealing with a related but more general issue).

Checklist

• The commits follow the conventional commits specification.
• You ran the linters with prek.
• You wrote unit test to demonstrate the bug you are fixing, or to stress the feature you are bringing.
• You reached 100% of code coverage on the code you edited, without abusive use of pragma: no cover
• If this PR is about a new feature, or a behavior change, you have updated the documentation accordingly. (N/A)

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