Add commit of link, layout, and public key files to cargo-in-toto-sign

[danbev]

Currently, running cargo-in-toto-sign will generate the step link files, the layout file, and the public key which will be stored in the sscs/in-toto/artifacts/branch_name (by default).
These files should be checked in, and they will later be used by cargo-in-toto-verify.

This issue suggests adding the files to git and committing as part of cargo-in-toto-sign to save users of the tool from the additional step. This could also remove old files that are no longer needed.