Set up gitconfig extra properties from the specified secret

[vinokurig]

What does this PR do?

Add a way to setup extra properties to the dev-container's system scope gitconfig file (etc/gitconfig).

What issues does this PR fix or reference?

eclipse-che/che#23306

Is it tested? How?

1. Add a secret to the user namespace with the next content:

kind: Secret
apiVersion: v1
metadata:
  name: devworkspace-gitconfig-extra-properties
  labels:
    controller.devfile.io/mount-to-devworkspace: 'true'
    controller.devfile.io/watch-secret: 'true'
stringData:
  http: extraHeader = some-value
type: Opaque

2. Start a dev-workspace and cat the etc/gitconfig file.

See: the gitconfig file contains the extraHeader property:

...
[http]
	extraHeader = some-value

run git config --list and see:

...
http.extraheader=some-value

PR Checklist

• E2E tests pass (when PR is ready, comment /test v8-devworkspace-operator-e2e, v8-che-happy-path to trigger)
  • v8-devworkspace-operator-e2e: DevWorkspace e2e test
  • v8-che-happy-path: Happy path for verification integration with Che

[openshift-ci]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: vinokurig
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[openshift-ci]

Hi @vinokurig. Thanks for your PR.

I'm waiting for a devfile member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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