MergeGatewayResources/MergeIR should merge specifics in HTTPRouteIR and ServiceIR like mergeGateways/mergeGatewayContexts

[sawsa307]

What would you like to be added:
Currently, MergeGatewayResources only performs merge on Gateways. This is done by merging multiple Gateways with the same namespace names into one Gateway, and make it inherit all Listeners and Addresses.

Merge processes are not performed for other resources like HTTPRoutes and TLSRoutes. Instances are grouped together using map.Copy(), which will overwrite the previous instance if there are more than one sharing the same key/Namespaced name. I believe we haven't run into this situation and don't have needs to do merge on them.

Once we figure we could have some resource(specifically HTTPRoute and Service) with multiple instances sharing the same namespaces, merge needs to be performed in a reasonable manner, and the merge should also consider how to merge specifics in HTTPRouteIR and ServiceIR.

Why this is needed:
Enhance merge operations on HTTPRouteIR and ServiceIR. If there can be multiple instances of HTTPRoute and Service with the same namespaced name, map.Copy() will lead to incomplete merge result.

[sawsa307]

/cc @LiorLieberman

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