internal/transport: validate per-RPC credential metadata (#9202)

Fixes #9198

Validate metadata returned by transport-level and call-level
`PerRPCCredentials` before appending it to HTTP/2 request headers. The
validation happens after keys are lowercased, matching the existing
outgoing metadata rules used for application metadata.

The regression tests cover invalid credential keys and values for both
dial-level and call-level credentials.

RELEASE NOTES:
* credentials: Invalid metadata returned by per-RPC credentials is
rejected before HTTP/2 request headers are created.