Config: Add support for SVN Mirror

[sbixl]

Is your feature request related to a problem? Please describe.

We currently face the issue that all of our SVN servers are being decommissioned and a read-only copy of the repository will remain available on a network drive.

To ensure that historical revisions remain build able, it would be very helpful to support an alternative mirror for the SVN repository in the system wide default.yaml configuration.

Unfortunately, the current fallback mirror feature only supports the URL SCM type and does not work with SVN repositories.

Describe the solution you'd like

Add support of the SCM Type SVN for the Mirror Feature.

Example:

fallbackMirror:
    - scm: svn
      url: "https://<my-svn-root-address>/svn/(.*)"
      mirror: "file://<my-network-drive>/Dir1/SVN_Backup/\\1"

Describe alternatives you've considered

A possible alternative would have been to use a Bob variable in the URL definition that contains the root address of the SVN repository, allowing it to be adjusted centrally via the default.yaml file. Unfortunately, this particular recipe was not implemented that way.

At the moment, I cannot think of any other practical solution. Without support for SVN repositories in the fallback mirror feature, or a configurable repository root URL, there seems to be no straightforward way to redirect existing repository references to the read-only mirror while keeping older revisions build-able.

[jkloetzke]

Sounds reasonable.

The reason why mirrors are only implemented for deterministic URL SCMs so far is that they are stateless. For everything else the chosen mirror must be saved per checkout somehow. But given how simple the SVN SCM is, that looks not too hard.