Python: Expose verify_peer_callback for Custom TLS Certificate Verification

[aibrahem]

Proposes adding an optional verify_peer_callback parameter to
grpc.ssl_channel_credentials() and grpc.ssl_server_credentials()
in Python, backed by the existing C-core
grpc_tls_certificate_verifier_external API.

Addresses the long-standing request in
grpc/grpc#32635 (labeled help wanted).

Earlier attempt in grpc/grpc#12656 stalled
on async/interpreter-lock concerns; this proposal starts with
synchronous mode, which the C-core API supports, and leaves async
as a future extension.

A working prototype exists locally with 10 unit tests (passing on
Bazel). I posted the design questions at grpc/grpc#32635 a week ago
but received only non-maintainer feedback, so I'm submitting the
gRFC to start the formal review process. Open to redirecting the
API shape based on reviewer input.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: aibrahem / name: Abdelrahman Ibrahem (db68199)