Add non-streaming response to approximate prefix cache.

[zetxqx]

What type of PR is this?

/kind feature

What this PR does / why we need it:

Add non streaming response to prefix cache, several points to pay attention:

1. This is only for non-streaming response, streaming response will be added in a followup PR.
2. The PR is based on Break PostResponse requestcontrol plugin into 3 separate plugins to add streamed request functionality #1661

• Also changed the ResponseComplete to accept a *types.LLMResponse which has richer structure.

4. The response will be concatenated to the previous request.
5. The encoding of ChatCompletions messages for prefix caching is now standardized via MarshalMessagesToJSON to improve caching accuracy for multi-turn conversations.
6. Currently only the first choice and the content will be prefix cached. (not role is added but could be a follow up)

Which issue(s) this PR fixes:

Fixes Partially #971

Does this PR introduce a user-facing change?:

[k8s-ci-robot]

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[zetxqx]

/assign @liu-cong

[k8s-ci-robot]

PR needs rebase.

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