Consider a common/unified full-text search API

Most databases have some sort of full-text search story, which typically involves linguistic intelligence and specialized indexing to provide fast, language-aware textual search. EF doesn't currently have a common story for this - we could introduce that here.

This would essentially be a common EF.Functions LINQ query stub, usable across all providers which support full-text search (but we may include common metadata as well, if that's called for). Do a cross-provider investigation to see what makes sense here.

Note: a major reason to do this is to provide a common/unified **hybrid** search story in EF, which combines both vector and full-text search (via e.g. RRF). Note that this is similar to #36350, which tracks a common/unified **vector** search API.

We are currently considering this as a stretch goal for 11.