Add external auth support

[dekkers]

This adds external auth support using Django remote user middleware. The header to be used can be configured using the REMOTE_USER_HEADER environment variable. The REMOTE_USER_FALLBACK variable can be set to enable the fallback to the normal model based auth backend.

I also changed the settings to only enable the OpenTelemetry middleware when SPAN_EXPORT_GRPC_ENDPOINT is set.

Fixes #963

[Donnype]

When I log in and try to change the language I get the following:

[underdarknl]

When I log in and try to change the language I get the following:

I'm still having issues with the notion that we need to do a POST for changing a language. But that's unrelated to this issue.
I'm guessing somewhere along the way the cookies get lost due to some https / secure cookie flags or allowed domains not being set correctly.

[dekkers]

When I log in and try to change the language I get the following:

I'm still having issues with the notion that we need to do a POST for changing a language. But that's unrelated to this issue. I'm guessing somewhere along the way the cookies get lost due to some https / secure cookie flags or allowed domains not being set correctly.

The problem is #1025 and unrelated to this PR. I added CSRF_TRUSTED_ORIGINS to the settings on the test server and that fixes the problem.

[Darwinkel]

Checklist for QA:

• I have checked out this branch, and successfully ran a fresh make reset.
• I confirmed that there are no unintended functional regressions in this branch:
  • I have managed to pass the onboarding flow
  • Objects and Findings are created properly
  • Tasks are created and completed properly
• I confirmed that the PR's advertised feature or hotfix works as intended.

I personally have no external authentication services running, so I can't actually test remote user support.

What works:

• Non-remote (model) user management still works

What doesn't work:

• kattest01.openkat.dev throws a 500 after attempting to login ;(