[gRPC](1/N) gRPC-in-gRPC-out support for Generate non-streaming

[zetxqx]

What type of PR is this?

/kind feature

What this PR does / why we need it:
Initial support for gRPC-in-gRPC-out non streaming Generate API.

Key changes:

1. copy the vllm proto definition and generate the client code to pkg/epp/api folder
2. Modify the epp/handlers/server.go flow for gRPC-in-gRPC-out, key difference are:
   • when receiving the gRPC request body, parse it using the new codec pkg. and populate the reqCtx.SchedulingRequestBody
   • determine the response endOfStream using trailer instead of depending on the EndOFstream

I tested the whole flow is working by using custom GKE controller.
The following is an successful result using test go code here: https://github.com/zetxqx/gateway-api-inference-extension/blob/c311c6f3097ee3337ccbe39714d6f649a1c3cba0/pkg/epp/grpc/examples/client/main.go

❯ go run ./pkg/epp/grpc/examples/client --target-address "<ip>:443"

--- Generate (Non-Streaming) ---
Complete Response (IDs): [100 101 102 103 104]
Complete Response (Text): <100><101><102><103><104>
Finish Reason: stop

Which issue(s) this PR fixes:

Fixes partially # #2166

Does this PR introduce a user-facing change?:

Add initial gRPC-in-gRPC-out for non streaming Generate RPC

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[ahg-g]

This is great, but I think we should take a step back and define a pluggable interface for request parsing. I created #2295.

I strongly suggest we first define the interface, then add the openAI http format as the first plugin, then we add the grpc one.

[ahg-g]

We should ask the vllm project to have the go generated code in vllm repo so we just import instead of copying the whole thing. We don't copy the ext-proc protobufs for example.

[ahg-g]

until then, we can keep this, but I would like to have this support exposed and implemented as a plugin, and so I would expect this proto definition to be stored with the plugin pkg.

[ahg-g]

Another thing, how do we keep the generated code up-to-date? I expected to see a makefile rule or something like that (similar to the CRD generation rule).

[ahg-g]

are those headers standard? is there a link we can reference here?

[RyanRosario]

/cc @RyanRosario

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@zetxqx: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gateway-api-inference-extension-verify-main	2b20cc1	link	true	/test pull-gateway-api-inference-extension-verify-main

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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