Add Git Credentials Helper as master source

[kwin]

There are lots of git credentials helper listed in https://git-scm.com/doc/credential-helpers.

As all have a common way of how they are parameterized and return credentials they can be added as master source as well.

As Git Credential helpers always use a URL as lookup key one would need to use an artificial URL to lookup the correct entry. In addition one would need to add the filename of the credential helper binary.