Allow string substitution in scmOverrides replace section

[jkloetzke]

Is your feature request related to a problem? Please describe.
In #665 it got clear that the following does not work at all:

layersScmOverrides:
-   match:
        ...
    replace:
        url:
            pattern: https://MY_PROD_GITLAB/(.+)
            replacement: https://gitlab-ci-token:${CI_JOB_TOKEN}@MY_TEST_GITLAB/\1

Describe the solution you'd like
Allow to opt-in for string substitution in replacement keys.

[MarcKe]

Why would you make it optional?

[jkloetzke]

Why would you make it optional?

Simply backwards compatibility. If an existing project has a $ in the replacement, it would break because that would be interpreted as a variable replacement now. Likewise, the backreference \1 will break because the variable substitution treats \ as an escape character (e.g. \$ to produce a literal $). So with variable substitution, the back reference must be written as \\1.

There are two ways to solve this: via a policy or an additional per-override setting. I'm more inclined to the latter because the policy is a global switch of the project but overrides are usually user supplied.