feat: add ability to pass user flags to python interpreter

[muravev-vasilii]

This change is meant to demonstrate the proposed update for the following feature request: #436

---

Changes are visible to end-users: yes/no

Users can influence behaviour of interpreter by setting flags via setting env variable RULES_PY_USER_INTERPRETER_FLAGS

Test plan

• Manual testing;

Ran bazel target with

RULES_PY_USER_INTERPRETER_FLAGS="-m debugpy --listen 12345 --wait-for-client" bazel run //path/to/rules_py/py_binary/target

From VSCode side:

• installed Python Debugger extension
• updated launch.json with

{
    "version": "0.2.0",
    "configurations": [
        {
            "name": "Python Debugger: Attach",
            "type": "debugpy",
            "request": "attach",
            "connect": {
              "host": "localhost",
              "port": 12345
            }
          }
    ]
}

Used Run&Debug tab in VSCode to set breakpoints, connect to program and debug

[CLAassistant]

All committers have signed the CLA.

[mnoah1]

Also, one other change for the debug use case is that the run template should set the PYDEVD_RESOLVE_SYMLINKS environment variable to true before starting debugpy. This resolves symlinks when sending paths to the client. Without this set, VS Code will open any paths within the Bazel cache as their own tab instead of showing the breakpoint in the original source file. More info: microsoft/debugpy#743