Extend HTTP API methods to match current proto and clean up Client

[FZambia]

Summary

Bring Client.php in line with Centrifugo's current HTTP API and clean up a few small issues ahead of v6.0.3.

Extended existing methods with new optional parameters from the proto

• publish / broadcast: tags, idempotency_key, delta, version, version_epoch
• subscribe: expire_at, info, data, recover_since, override, session
• unsubscribe: session
• disconnect: disconnect (code/reason), whitelist, session

All new parameters are positional optional arguments at the end with sensible defaults, and are only included in the JSON payload when set to a non-default value — so existing callers see byte-identical request bodies.

Out of scope (deliberately): adding entirely new RPCs not previously exposed (rpc, refresh, connections, push/device/user-status/etc.). Worth a follow-up.

Drive-by fixes

• setSafety docblock — @param was incorrectly labelled $caPath.
• _json_last_error_msg — the function_exists('json_last_error_msg') fallback table was dead code on any PHP version this library supports (composer requires >= 7.0; json_last_error_msg exists since PHP 5.5). Body shrunk to a one-liner; public signature preserved for BC.
• request() — if (!$this->safety) { ... } elseif ($this->safety) { ... } collapsed to plain else.
• getUrl() — rtrim($this->url, '/') so passing a trailing-slash URL like http://host/api/ no longer yields http://host/api//publish.
• README — "All available API methods" block was missing subscribe and batch.

Test plan

• Existing tests/ClientTest.php still passes against latest Centrifugo (no call sites changed; new params are optional).
• Manual sanity check: payloads for default-arg calls of publish / broadcast / subscribe / unsubscribe / disconnect are byte-identical to v6.0.2.
• Optional: add a test exercising one of the new fields (e.g. tags on publish, session on disconnect).

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