Cache database repository objects for API requests

Context

In #449 (closed) we observed that repositoryStore.FindByPath and repositoryStore.CreateOrFindByPath are mildly expensive operations, and since repository scoped requests only deal with a single repository, we should avoid instantiating this repository object more than once.

Solutions

Pass via the Context

Instantiating this object before the handlers and passing it along via the context would discourage extra calls to repositoryStore.FindByPath https://gitlab.com/gitlab-org/container-registry/-/blob/master/registry/handlers/app.go#L976, but sometimes we'll wish to create the repository if it doesn't exist, and other times we need to error out if it does not exist.

Create a memoized repository store

Saving the result of repositoryStore.FindByPath and repositoryStore.CreateOrFindByPath allows us to only make the call once per request, while allowing us to continue using the previous logic in the callers.