[installer-tests] Add SCM authentication for integration tests

[nandajavarma]

Description

This PR integrates the SCM authentication when running integration tests. The changes included here are the follows:

• Using the GITHUB_SCM_OAUTH env var to provide the secret in core-dev cluster that stores the github token for github app
• The secret will be appended with the callBackURL. This URL would point to an nginx server we have setup that will redirect to the correct subdomian specified using state query parameter
• The above changes are added only when integration tests are run, otherwise we expect the users to have the opportunity to connect to their preferred SCM
• The PR also changes one of the existing annotation skipTests to runTests. This is mostly because, we don't want the test running a default behavior, but an explicitly specified one.
• The PR also updates the slack alert channel to be the corresponding team channels since potentially the fix in this PR should make the integration tests work.

Related Issue(s)

Fixes #12870

How to test

You can run the command:

werft run github -j .werft/k3s-installer-tests.yaml -a runTests=true

Release Notes

NONE

Documentation

Werft options:

• /werft with-preview

[nandajavarma]

/werft run with-sh-preview

👍 started the job as gitpod-build-nvn-githib-scm.4
(with .werft/ from main)

[nandajavarma]

Fixed merge conflicts

[mrsimonemms]

/hold

One question, but happy for this to be merged as-is

[nandajavarma]

/unhold