feat: Support "echo" for TensorRT-LLM models in OpenAI API frontend v1/completions

[yinggeh]

What does the PR do?

Support "echo" for TensorRT-LLM models in OpenAI API frontend v1/completions
Note: Only limited TRT-LLM models support "echo" (inflight_batcher_llm, disaggregated_serving, llmapi)

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
• Populated github labels field
• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
• Verified copyright is correct on all changed files.
• Added succinct git squash message before merging ref.
• All template sections are filled out.
• Optional: Additional screenshots for behavior/output changes with before/after.

Commit Type:

Check the conventional commit type
box here and add the label to the github PR.

• feat

Related PRs:

Where should the reviewer start?

Test plan:

• CI Pipeline ID:
  38760391

Caveats:

Background

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• closes GitHub issue: #xxx

[whoisj]

LGTM