Adding Settings for Secure Files Object Storage

What does this Merge Request do and why?

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

The feature is designed to support folks who need a simple way to add binary files to their CI pipelines. The use case I'm focused on 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).

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

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.