Place repositories (git/oci) behind the Repository interface and use a Factory to create instances of implementations

[liamfallon]

This PR refactors the existing Porch code to make it easier to introduce a database cache.

It reinforces the structure of the code so that repository implementations (git/oci) are behind the interface and are only used by calling the interface. The externalrepo package is introduced as a package that contains "external" repository implementations. The following list details the refactors:

• A new pkg/externalrepo package is introduced, this package contains a factory for creating implementations of the Repository type that store packages that are source or target packages for Porch (such as git and oci), that is Repository implementations that are not Porch internal caches
• The pkg/git' package is moved to pkg/externalrepo/git`
• The pkg/oci' package is moved to pkg/externalrepo/oci`
• Types for the repoimpl factory are in pkg/externalrepo/types (to avoid circular imports)
• The ExternalRepoFactory interface is introduced to create instances of repos (git or oci today)
• The CreateExternalRepo function in pkg/externalrepo/externalrepo.go creates an instance of a repo based on the incoming configuration, calling the factory in either git or oci
• The cache options are restructured with a contained ExternalRepoOptions struct containing the options that the ExternalRepo implementations use but the cache does not use
• The memory cache is renamed as the CR (Custom Resource) cache to better reflect how it works
• A CacheFactory interface is introduced for creating cache implementations, where the implementation of the cache is behind the factory interlace
• A CacheFactory implementation is added to the CR cache cache.

[liamfallon]

/assign @kispaljr @efiacor @kushnaidu @JamesMcDermott @Catalin-Stratulat-Ericsson

[efiacor]

Few minor comments.
Do we need some unit test coverage on the new repoimpl stuff? Not sure if it's covered currently.

[liamfallon]

Few minor comments. Do we need some unit test coverage on the new repoimpl stuff? Not sure if it's covered currently.

yes, I'll do the unit tests in a later PR.

[kushnaidu]

Looks great Liam.

[liamfallon]

/test presubmit-nephio-go-test

[efiacor]

/approve
/lgtm

[nephio-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: efiacor, liamfallon

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [efiacor,liamfallon]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment