feat: provide universal embedabble attributes for element

[kdziedzic68]

Feature description

1. Base Element class should have optional embeddable_text and embeddable_image properties defining the content for the two currently supported embedding types
2. To create the embeddable_text property should we should simply rename the current key property to embeddable_text
3. The implementation embeddable_image in the base Element class should simply always return None. The ImageElement class should override this implementation to return the bytes of the image instead.
4. Proper adjustments should be made in insert_element method of DocumentSearch

Motivation

Current implementation assumes that emedded images are instances of ImageElement. we should make it more flexible to potentially support different classes

Additional context

No response