Expand API to include Signing
### Description
Visibility of commit signatures has been added previously alongside the capability of tying these to a GitLab user for verification. It would be nice if the API (specifically CreateFile, UpdateFile, and Commits) could be expanded to allow for the inclusion of a signature.
### Proposal
Add an additional attribute to the Repositories Files and Commits API methods to allow for a private key to be specified to allow the commit to be signed.
Use cases for this include automated accounts used in conjunction with tools such as [Renovate](https://renovateapp.com/) where it is desirable to indicate they have been generated from an authorative source.
### Links / references
issue