KIP-1139 — Add support for OAuth jwt-bearer grant type

Accepted Kafka 4.1 Security

Adds support for the OAuth 2.0 `urn:ietf:params:oauth:grant-type:jwt-bearer` grant type (RFC 7523) in Kafka clients alongside the existing `client_credentials` grant, allowing authentication via a signed JWT assertion instead of a plaintext secret. The `client_credentials` grant requires embedding plaintext secrets in client configuration, which many organizations and cloud providers prohibit for security compliance.

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