Add CEL Validation to disable MergeGateways from being set when EnvoyProxy targets a Gateway

[arkodg]

          should we add validation to not allow setting MergeGateways on EnvoyProxy when attaching it to the Gateway?

Originally posted by @cnvergence in #3532 (review)

[haoqixu]

Can we achieve it via CEL? 🤔

[cnvergence]

I can take a look at it :)