Add AgentgatewayParameters

[josh-pritchard]

AgentgatewayParameters are going to be part of the new agentgateway.dev Group so agentgateway and the envoy based gateway have completely seperate UX for install and usage. We think it will be clearer to users if there is an obvious separation between API groups & we can avoid API surface where there are fields specific to each implementation in the same CRD.

[chandler-solo]

#13007 and #13018 are directly aimed at this.

[lgadban]

Closed via #13018