Text layer extraction and storage

[kirillt]

For resources of kind "Document", it would be useful to extract and store text from them. E.g. for PDF resources, text layer should be similar to what is emitted by Linux utility pdftotext. The text layer can be used later for filtering resources by specified text in content, or for various text analytics (e.g. counting words).

[kirillt]

Rust side:

• Extract text layer from PDF ark-android#57

Android side:

• create bindings to the Rust function
• modify https://github.com/ARK-Builders/arklib-android/blob/main/lib/src/main/java/dev/arkbuilders/arklib/data/metadata/extractor/DocumentMetadataExtractor.kt and call the Rust function on PDF files
• enable new feature in Navigator