Allow for multiple vector fields in AzureSearch

[bbfred]

Checked

• I searched existing ideas and did not find a similar one
• I added a very descriptive title
• I've clearly described the feature request and motivation for it

Feature request

Azure Search allows to run query across multiple vector fields. This can be done by using VectorizableTextQuery with multiple fields. In order to be able to take advantage of this feature, when performing a search with langchain we should be able to specify the search fields. Currently it's a const FIELDS_CONTENT_VECTOR.

Multiple search field vectorisation.
vector_query = VectorizableTextQuery(text=query, k_nearest_neighbors=3, fields="contentVector, titleVector")

Motivation

The motivation is to be able to perform search on a document and its title at the same time for example. This improves search results.

Proposal (If applicable)

I would propose to have the option to pass search fields as a kwargs to simple_search for example.