Draft: Add support for git credentials

What does this MR do?

This commit add a new internal git-credential command to the gitlab-runner and gitlab-runner-helper binaries and a new feature flag FF_USE_GIT_CREDENTIALS.

When enabled, repository credentials will be provided by using the git credential mechanism.

Why was this MR needed?

This MR allows cloning of repositories hosted on a Kerberized GitLab instance without a dedicated SPNEGO port.

What's the best way to test this MR?

1. Enable Kerberos on a GitLab instance, no need for a functioning Kerberos configuration, only setting kerberos.enabled = true should be enough. Do not enable kerberos.use_dedicated_port.
2. Enable the FF_USE_GIT_CREDENTIALS feature flag
3. Start a pipeline on any repository

What are the relevant issue numbers?

Fixes: gitlab#49