api: do not embed HTTPRoute as-is

[mathetake]

This is a follow up on #39, and stops embedding HTTPRoute as-is.
The rationale here is somewhat complicated; We have to know which
backend we route traffics to before Envoy decides the routing as
we need to do perform the transformation etc. Hence, we need to
implement the routing logics (e.g. header matching, weights, etc.)
by ourselves, and cannot rely on the native router code path in Envoy.
That results in us only being able to support only a small subset of
HTTPRoute functionality as well as we cannot simply use the embedded
HTTPRoute as a "base" in the translation. If we embed HTTPRoute here,
it would create an impression that all HTTPRoute functionality can be
supported here no matter how much we document otherwise.

We discussed about this issue, and reached the consensus that we should
have our own LLMRoute rule definition and only have the fields that
actually can be supported.

The example API would look like;

kind: LLMRoute
metadata:
  name: 
  namespace: default
spec:
  inputSchema:
    schema: OpenAI
  rules:
    - matches:
      - headers:
        - type: Exact
          name: x-envoy-ai-gateway-llm-model
          value: llama3-70b
      backendRefs:
        - name: kserve
          weight: 20
        - name: aws-bedrock
          weight: 80

[mathetake]

after this i finally will work on implementing the initial controller/extproc code