Backend: Add Remaining TCF Overlay Contents

[pattisdr]

Is your feature request related to a specific problem?

This PR adds the starting point for a TCF overlay: #3804, with details about purposes and special purposes returned, some minimal info about vendors, and a framework for features, and special features but no data returned yet.

We don't yet have all the details we need on systems yet to be able to populate this.

Describe the solution you'd like

Big picture, we need to add vendor, features, special features, and custom purposes to the TCF overlay:

• Pull vendor field directly from System instead of from attached integration
• Populate Vendor sections with top-level description, is_gvl, etc. Also populate Vendor special feature, feature, and custom purpose embedded sections
• Query for matching features and special features on systems and return those contents in the TCF overlay
• Populate is_gvl information

Describe alternatives you've considered, if any

A description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.

[pattisdr]

Further items:

• Make sure embedded line items have sufficient detail, right now it's generally just the title
• Reference Feature schemas from fideslang and remove schema in fides