[Integration Tests] Fix flakiness by removing external host dependencies

[abhinavsingh]

Currently our integration tests use httpbin and other services. This unfortunately results in flaky behavior where upstream server can throw any kind of error. We must bake in replica's of upstream services within proxy.py test module to replicate upstream services. Then integration tests will need no internet connection itself.

Example see how upstream server can cause flakiness

• https://github.com/abhinavsingh/proxy.py/runs/4817497950?check_suite_focus=true
• https://github.com/abhinavsingh/proxy.py/runs/4906368525?check_suite_focus=true