[sync] Abstract storage API to access storage

[radumarias]

Have an abstraction layer to interact with the local filesystem and use it in EncryptedFs and crypto module when accessing the actual local files in data-dir. The benefit is that we can then run the core on several platforms, and this layer will handle the particular interaction with each platform's storage.
Like having implementations for desktop, WASM, and mobile that handle each platform's particularities of each platform.

The API should be compatible with std lib file API from https://doc.rust-lang.org/std/fs/struct.File.html , https://doc.rust-lang.org/stable/std/fs/ and https://doc.rust-lang.org/stable/std/io/

Structure of crypto module

crypto::storage::fs::OpenOptions
crypto::storage::fs::File
crypto::storage::fs
crypto::storage::io
crypto::storage::path::Path. This is because the existing method uses local fs
crypto::storage::path::PathBuf. This is because the existing method uses local fs

Implementations

Use the glue logic from src/mount.rs where you have structs with the same name in different modules, and then based on the future flags, use the corresponding one, by default, the std.

std lib File API to save directly to the local filesystem

crypto::storage::provider::std::fs::OpenOptions
crypto::storage::provider::std::fs::File
crypto::storage::provider::std::fs
crypto::storage::provider::std::io
crypto::storage::provider::std::path::Path
crypto::storage::provider::std::path::PathBuf

Flow is like this

lib-users > crypto::[fs, fs::OpenOptions, fs::File, io] > EncryptedFs -> crypto::storage::[fs, fs::OpenOptions, fs::File, io]

This issue corresponds to encrypted_storage::[fs, fs::File, io].

#111