tests: Move httpx under toxgen

[sentrivana]

• Move httpx under toxgen
  • This is not straightforward since we depend on pytest-httpx which has a very weird strict relationship to which httpx versions it supports
  • In addition to that, some pytest-httpx versions will not install on Python 3.8 despite the corresponding httpx versions being compatible -- added a new way to filter out these scenarios
• This also removes the -latest Networking group as the httpx test suite was the last Networking test suite that wasn't completely pinned
• Also move gcp to the permanent part of the ignore list as it doesn't fit the toxgen mold

Ref #4506

[codecov]

✅ All tests passed in 2608.32s