validate GameServerBuild objects

[dgkanatsios]

Currently, the only validation we're doing on GameServerBuild CRD instance is via OpenAPI v3 schema. However, this provides only some naive validation and we should work on validating more things like:

• standingBy number is <= max
• when creating a new GameServerBuild, there is NOT an existing GameServerBuild with a different name but the same BuildID. We can check for existing GameServerBuilds with the same BuildID using the metadata-only client as described here
• validate that hostPort is empty on all containers on the GameServerBuild for ports that are in the portsToExpose array
• validate that name is included for ports that are on the portsToExpose array
• validate that all ports on the portsToExpose array exist in the pod container spec

We could use validation webhooks.