[FEATURE] enable handling synonyms in neural ingestion pipelines

[asfoorial]

Handling synonyms is like an ABC thing when it comes to semantic search, however the current neural search plugin does not handle synonyms.

I suggest a feature to enable that out of the box without really going through the expense of fine-tuning embedding model. One simple way to do it is by simply replacing a word with both the word and its synonyms before generate a vector for them. The source text remains the same but the embedding gets to reflect the synonyms. The option can also be disabled in case the embedding model already understands synonyms.

Example:

Src="OS is amazing"
dst="[[Open Search]]os is amazing"
vec= getEmbedding(dst)

POST test/_doc/1
{"content":src,
"Vec":vec

}