feat(document-search): add location metadata to Element(s)

[mhordynski]

Feature description

Some providers (like unstructured) provide additional information about location of the element (chunk) in the source document. It may be something like page number, XY location, location of the first character, etc.

Element base type should have an extra optional field called location with that information stored.

Motivation

With location being stored in returned elements, ragbits users would be able to more accuratly point document reference in rag use-cases

Additional context

No response