Add a Database Cache to Porch

[liamfallon]

Currently Porch uses etcd to cache the metadata for packages. This work will add a new database cache that will cache the entire package including metadata to a database.

With the DB cache, Porch will access Git on the "Approve" and "Delete" actions, so there will be less access required to Git. In the DB cache, we will only need to lock on packages, so simultaneous access to different packages will be possible as long as you are not doing an "Approve" or a "Delete".

The DB cache:

• enables HA on Porch (even across k8s clusters)
• improves data consistency
• makes operations more performant, reads no longer go to git and writes do reads before writing so they will be faster too
• reduces the complexity of the Porch code

This work will be carried out in phases:

• Rename GIT CA Bundles to be non repo implementation specific Rename GitCaBundle to UserDefinedCaBundle porch#169
• Place repositories (git/oci) behind the Repository interface and use a Factory to create instances of implementations Place repositories (git/oci) behind the Repository interface and use a Factory to create instances of implementations porch#171
• Place the existing Cache implementation behind an interface so that cache implementations are opaque to the rest of the Porch code
• Implement the DB cache up to the point that it can handle Package Revisions up to state "Proposed"
• Implement pushing of "Approved" packages from the DB cache into Git
• Implement DB transactions on Porch operations
• Unit tests for all code behind the Repository implementation interface
• Unit tests for all code behind the Cache interface
• e2e tests updated for the DB cache