ZOOKEEPER-4990: accept ssl.* in client config

[seekskyworld]

Summary:

• Allow ZKClientConfig to fall back to server-style ssl.* keys when zookeeper.ssl.* is absent.
• Add test coverage for the fallback and precedence behavior.
• Document zkCli TLS client.properties usage.

Motivation:
Client config files copied from zoo.cfg (ssl.* keys) don’t populate zookeeper.ssl.* today, which can lead to TLS handshake failures. This makes zkCli client configuration compatible with server-style keys while keeping zookeeper.ssl.* as the preferred form.

Validation:

• mvn -pl zookeeper-server -Dtest=ZKClientConfigTest test (Passed)