v1/api: support adding content template to a blueprint

[xbhouse]

To add support for adding a content template to a blueprint, this PR does a few things:

• Adds a new field (template) to the blueprint customizations, which holds the template UUID
• Saves the template UUID and snapshot date from the template to the blueprint on creation
• Resolves custom repositories in the template at compose time
• Adds another content sources mock for the unit tests

Ticket here: https://issues.redhat.com/browse/HMS-5319

Open question(s):

• How should the Red Hat repositories in the template be handled? Currently I've filtered out and added only the custom repos to the blueprint, I'm not sure if this is what we want though 🤔
• If a template is added to a blueprint, should additional custom repositories and packages be able to be added? 💭