Enable registry read-only mode to be triggered via CR

Story: As a administrator I want to set Quay into read-only mode conveniently via the operator so that I don't have to carry out a lot of manual steps in order to create a maintenance window for upgrades of a geo-replication setup.

Background: Setting Quay into read-only mode is quite involved. The process is described here: https://access.redhat.com/articles/5411111 - while we should probably also invest in making this process simpler to begin with, this is an opportunity for the operator to hide this complexity and provide a simple control knob to enable disable / readonly mode.

Acceptance criteria:

• the operator's CRD introduces a new toggle (outside the managed components) to set the entire registry into read-only mode and get it out of read-only mode
• during read-only mode the status block of the CR updated to reflect this particualr condition
• the admin can set the new toggle to "true" which will transition the registry into read-only mode and should cause the registry UI to display a banner appropriately
• the process to get the registry into read-only is fully automated
• if transition to read-only mode fails for any reason the status block of the CR should get updated accordingly with a description of the failure and registry should return to read-write mode
• upon setting the new toggle to "false" the registry will transition back into read-write mode
• if the registry is in a transient state into or out of read-only mode the status block of the CR should reflect that