Implement FileReadableKVState

[IvanKavaldzhiev]

Problem

We need to add a new type of database accessor to read system files data, so that we have specific ReadableKVState instance for this type of data.

Solution

Add FileDatabaseAccessor class that reads system files and converts them to File PBJ type.

Add unit tests to cover all cases including historical reads and correct mapping for all fields.

Alternatives

No response