feat: Add AdkWebClient service

[kalenkevich]

Link to Issue or Description of Change

Add AdkWebClient class which is a client class for the ADK Web server. Useful while working with integrations/conformance tests.

Note: Also we should think about exposing that as part of the public API.

Testing Plan

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Unit Tests:

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Checklist

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[ScottMansfield]

Sure, this seems useful for integration testing.