KIP-651 — Support PEM format for SSL certificates and private key

Accepted Kafka 2.7 Security

Adds support for PEM-encoded certificates and private keys directly in Kafka SSL configuration via new `ssl.keystore.type=PEM` and inline `ssl.keystore.certificate.chain`/`ssl.keystore.key` configs, eliminating the need to manage JKS or PKCS12 keystore files. Managing JKS/PKCS12 keystores in containerized and secret-management environments (Vault, Kubernetes Secrets) is cumbersome; PEM strings can be injected directly from environment variables or secret stores.

Details