Feature Request: Allow 3rd party repositories to set priority

[jrmcpeek]

Current state:

When adding a 3rd party repository for use a build time, there is no option to set a priority for the repository.

Issue:

Some repositories contain packages that conflict with each other, and depending on the packages defined in the blueprint, can have unexpected results as to which packages from which repositories are installed as dependencies.

Request:

Allow a priority field to be set when using 3rd party repositories, mapping to repository priority from DNF's perspective.

If not set, use the default priority of newly added repos.

Note: When adding custom repositories in a blueprint, this field is already present - but custom repositories in the blueprint cannot be used to pull packages at build time.

Benefit:

For image building scenarios making use of 3rd party repositories, we can control the priority for repositories when multiple repositories can provide a dependency.