[8.18] Adds EIS POST inference details.

[szabosteve]

Overview

Backport of #3940

[github-actions]

Following you can find the validation results for the APIs you have changed.

API	Status	Request	Response
inference.chat_completion_unified	⚪	Missing test	Missing test
inference.completion	⚪	Missing test	Missing test
inference.delete	⚪	Missing test	Missing test
inference.get	🟢	1/1	1/1
inference.post_eis_chat_completion	⚪	Missing test	Missing test
inference.put_eis	⚪	Missing test	Missing test
inference.put_mistral	🟠	Missing type	Missing type
inference.put_openai	⚪	Missing test	Missing test
inference.put_voyageai	⚪	Missing test	Missing test
inference.put_watsonx	⚪	Missing test	Missing test
inference.put	⚪	Missing test	Missing test
inference.rerank	⚪	Missing test	Missing test
inference.sparse_embedding	⚪	Missing test	Missing test
inference.stream_completion	⚪	Missing test	Missing test
inference.text_embedding	⚪	Missing test	Missing test
inference.update	⚪	Missing test	Missing test

You can validate these APIs yourself by using the make validate target.