pkgs/ok_http: Close the client (override close() from BaseClient)

[Anikate-De]

Items in this PR:

• Generated JNI Bindings for okhttp3.ConnectionPool (allows us to use the evictAll() method) and okhttp3.Dispatcher (for getting the ExecutorService)
• Override void close() from BaseClient
• Add testClose()
• Add testResponseHeaders() since all TCs are passing now that [http_client_conformance_tests] Add flag to skip tests for folded headers #1219 is closed (see pkgs/http_client_conformance_tests: add boolean flag supportsFoldedHeaders #1222)

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