Global pac configurations

As an admin, I'd like to set global configurations that apply to all repositories on the cluster so that I can enforce limits and configs across teams and namespaces.

The use-cases for global configs are:

• Enforce a max value for max-keep-runs so that developers cannot exceed that and can only set a value between 0 and the max number determined by the admin
• Set default max-keep-runs to apply to pipelineruns that don't have any value set

Acceptance Criteria

• Admin can set global max value for max-keep-runs
• Admin can set default max-keep-runs to apply to pipelineruns that don't have any value set