Allow using this gem as a library

[endorama]

Description

We are currently focusing on a POC to enable gem signing and verification through the gem command.

For the long term, I think would be meaningful to abstract this gem to allow usage as a library (i.e. require sigstore).
I don't think is something to be done in the short term, but I wanted to open this issue to discuss about it and eventually keep track of this enhancement for the future.

What do you think?