Allow route level body mutation

[xiaolin593]

Description:

Similar to route level header mutation, route level body mutation will allow user to specify what field to modify/override/replace.

For instance, when have multiple backends OpenAI and Azure stack together to serve GPT-4.1 traffic. route level body mutation allow user to specify service_tier value only for OpenAI but Azure which doesn't support service_tier.

Similar override will apply to Anthropic's service_tier override.

[xiaolin593]

@envoyproxy/ai-gateway-admins please assign the issue to me.

[mathetake]

This is exactly the issue I discussed with @tiswanso #1386 -- we reached the consensus that we should add a generic jsonpatch style stuff

[xiaolin593]

This is exactly the issue I discussed with @tiswanso #1386 -- we reached the consensus that we should add a generic jsonpatch style stuff

Can I start with top-level field mutation first ? @mathetake I have it work locally, addressing tests before open PR.

[xiaolin593]

Not sure if we need comprehensive jsonpatch as of now.

[mathetake]

I would definitely love to start with jsonpath rather than specifics, at least it should cover your case as well as @tiswanso's case altogether