feat: adds ci Secure Files settings

What does this MR do?

This change adds settings configuration to support the Secure Files mechanism discussed in gitlab-org/gitlab#346290 (closed).

The feature is designed to support folks who need a simple way to add binary files to their CI pipelines. The main use case here is in the code signing process for mobile teams, but this approach has been generalized to support other use cases as well.

This first iteration supports basic API interactions. Future iterations will support direct integration with the GitLab Runner, as well as a UI for file uploads and viewing (discussion here).

@darbyfrey recorded a demo of this feature as well: https://youtu.be/eK3FUskHfdo

Related issues

https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15370

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened 👉 gitlab-org/omnibus-gitlab!5857 (merged)