[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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[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: zetxqx
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