ES|QL - dense_vector approximate nearest neighbour search support

[carlosdelest]

Description

As dense_vector support is being addressed in #125783, ES|QL will be able to perform approximate nearest neighbour vector search.

The following tasks will be needed:

• kNN search function using vectors: ES|QL - kNN function initial support #127322
• ESQL - Add K mandatory param for KNN function #129763
• kNN uses LIMIT for setting k
• Prefiltering in kNN search
• kNN text search, using Inference
• kNN for semantic_text fields

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)