feat(linux): add simple_secure_storage_linux_portal package

[EchoEllet]

Fixes #16

Checklist

• Remove xdg_secret_portal_store_default and implement xdg_secret_portal_store using cipherlib to migrate away from cryptography ([linux] support XDG Desktop Secret Portal (org.freedesktop.portal.Secret) #16 (comment))
• Test this on a real Flutter Linux application using simple_secure_storage with a Flatpak sandbox
• Update the README of simple_secure_storage to reference simple_secure_storage_linux_portal and when to use it or how.
• Decide whether we should use the running Linux application ID or InitializationOptions.appName and/or InitializationOptions.namespace for building the file path of the stored secrets. I purpose Linux application ID + allowing it to be overridden using applicationIdOverride (see also this section).

Tip

Using direct access to the Secret Service API may result in
Flathub app submission not being approved.
New sandboxed applications should consider the XDG Desktop Portal Secret API when
available. This information can be documented in simple_secure_storage's README.

Dependencies

• package:xdg_desktop_portal: A native Dart client implementation of XDG Desktop portals. The secret portal provides a unique master secret for the running application. By design, it is not a complete high-level secure storage like the secret service API.
• package:xdg_secret_portal_store: A helper library for storing application secrets in an encrypted file using the master secret provided by the XDG Desktop Portal Secret API. Designed to complement package:xdg_desktop_portal (but does not depend directly on it).
• package:xdg_secret_portal_store_default: A default cryptographic implementation of package:xdg_secret_portal_store that uses package:cryptography. Can be removed.
• package:linux_application_id + package:xdg_directories: Used to build the file path for the file containing the encrypted stored secrets. These dependencies could be replaced with path_provider, but this would introduce more transitive dependencies. path_provider_linux depends on xdg_directories internally.