Allow placement of Gateways

[maleck13]

What

As a gateway admin, I want to be able to chose to place a gateway on a set of clusters from my available clusters .

option

use an exact match placement label

kuadrant.io/placement: "key=value"

at a future point we may need more than this simplistic placement logic but this is lightweight.
When a placement is added, the controller needs to figure out which clusters names (secrets) this is selecting. It then needs to add a new label something like: kuadrant.io/<cluster>/placement.decision: "true" which the sync agent will watch for

Done

• new placement label defined
• new logic to find the cluster the label matches
• new logic to add a placement decision to the gateway resource
• new unit tests

[maleck13]

An alternative here is to lean into the KCP ideas around placement
https://github.com/kcp-dev/kcp/blob/759f510165a012c28f24c2efbdfc2a258354a21b/config/crds/scheduling.kcp.io_placements.yaml

So allowing to target a resource type and place that resource type on clusters that match the label selectors