[Feature Request] Add support for faceting on joined reference fields

[mbombonato]

Description

Consider the following schemas:

{
  "name":  "books",
  "fields": [
    {"name": "title", "type": "string"},
    {"name": "author_id", "type": "string", "reference": "authors.id"}
  ]
}

{
  "name":  "authors",
  "fields": [
    {"name": "id", "type": "string"},
    {"name": "name", "type": "string"}
  ]
}

I'd like to request a facet for the field "name" of the referenced collection "authors"

{
	"collection": "books",
	"include_fields": "$authors(name)",
	"q": "famous"
	"facet_by": "$authors(name)"
}

Expected Behavior

Return the results with a facet for the field "authors.name"

Actual Behavior

{
	"message": "Could not find a facet field named `$authors` in the schema."
}

Metadata

Typesense Version: 26

OS: any

[mbombonato]

I'd like to add the importance of this feature. I was about to change my data model to use the new JOIN feature of the Typesense 26. This would save us a huge amount of indexing time and data storage. Now I have to wait until this feature is released, because we need faceting working for referenced collections :(

Adding support for JOINs was immense new feature, but still incomplete without the proper faceting support.