[Feature] Upgrade available endpoint / dependent upgrades

[SimonBaeumer]

Problem In ACS we have a client<>server architecture in which our server should always be ahead or equal in its version of the clients. Currently OLM auto-upgrades upgrade upgrade the client faster than the server, especially in our cloud environments this happens currently and need to support forward-compatible clients.

Solution Expose an endpoint on the operator which OLM can query which marks an operator as "ready for upgrade". In this scenario, our operator can implement dependent upgrades: "Only upgrade client when central XYZ". The endpoint can also expose the version the operator should upgrade to.

Example response:

{
  "upgradeVersion": "4.1.2-567ds2sg",
  "ready": true,
}