feat: support commit prefix in onboarding commit (#1868)

This PR adds the capability so that the onboarding commit message (“Add renovate.json”) will have a prefix if semanticCommits is explicitly set to true by the bot administrator.

e.g. “renovate a/b —semantic-commits=true —semantic-commit-type=foo” will result in the onboarding commit being “foobar: add renovate.json”.

Closes #1867