Support docker registry secrets for AppRepositories

[absoludity]

As part of implementing support for app repositories using private docker registries (#1256), we need to support linking docker registry secrets to AppRepositories. See design doc for more info and discussion.

• Update the AppRepository resource to support the additional field dockerRegistrySecrets (a list of strings containing the names of any secrets that should be made available by the repo)
• [UI] Enable selecting from existing dockerconfigjson secrets in the given namespace, as required for an AppRepository.
• [UI] Enable adding new dockerconfigjson secrets while creating/editing an AppRepository
• Ensuring that repos available in all namespaces cannot have docker registry secrets added (as the secrets are available only in the namespace of the app repository).