[FEEDS-1373] allow users to provide a custom OkHttpClient

[itsmeadi]

Summary

• Add new constructors on StreamHTTPClient and StreamSDKClient that accept a user-provided OkHttpClient, giving full control over connection pool, timeouts, proxy, SSL, and any other OkHttp setting.
• The SDK calls client.newBuilder() on the provided client, so the user's configuration is inherited while the SDK's required interceptors (auth, logging, headers) are still applied.
• Existing constructors are unchanged — fully backward-compatible.

Usage:

OkHttpClient customClient = new OkHttpClient.Builder()
    .connectionPool(new ConnectionPool(10, 120, TimeUnit.SECONDS))
    .connectTimeout(30, TimeUnit.SECONDS)
    .proxy(myProxy)
    .build();

var client = new StreamSDKClient(apiKey, apiSecret, customClient);

Test plan

• Verify all existing constructors still work as before
• Create an OkHttpClient with a custom connection pool and pass it to new StreamSDKClient(key, secret, client) — confirm the pool config is inherited
• Confirm SDK interceptors (auth header, api_key param, X-Stream-Client header) are present on outgoing requests
• Confirm logging interceptor is applied at default level

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