[ES|QL][Completion] Elastic managed LLM support for COMPLETION

[afoucret]

Elastic managed LLM in completion:

• https://github.com/elastic/eis-gateway/pull/838
• [Inference] Implementing the completion task type on EIS. #137677
• Introducing a new predefined completion endpoint (.gp-llm-v2-completion) #138420

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[elasticsearchmachine]

Pinging @elastic/search-relevance (Team:Search - Relevance)

[afoucret]

There is a new pre-defined completion endpoint .gp-llm-v2-completion that is usable with the COMPLETION command:

| COMPLETION prompt WITH { "inference_id": ".gp-llm-v2-completion"}