Add PYDEVD_REMOTE_ROOT support

[thejcannon]

This change intends to fix microsoft/debugpy#1130 by allowing for a env var (PYDEVD_REMOTE_ROOT) to unconditionally configure the remote root when set in the server's environment.

[thejcannon]

I couldn't find a good place to add a test for this. LMK where one belongs

[fabioz]

Thanks for taking a look at this.

Some notes:

1. There should be an integration test in tests_python/test_debugger_json.py (you can use another test as a base -- test_case_stop_async_iteration_exception has an example which customizes environment variables).
2. Most constants are set in _pydevd_bundle.pydevd_constants. I think this should be a constant there (initialized from that environment variable value).
3. If that variable is set it should be logged in pydevd._log_initial_info.

[thejcannon]

• Added integration test. Fails without changes, passes with
• now using a defined constant
• logging the constant if set

[thejcannon]

@fabioz wanna give this another look?

[thejcannon]

Bump

[thejcannon]

@fabioz are you able to take a look? 😄