This PR adds a minimal backup feature (#391)

Backup Service

• a BackupService loads and saves BackupData interfaces from a distant database.
• ports directory provides two BackupService implementations: BrowserSyncBackupService & GithubBackupService.
• a new background service has been add to the background-script.ts context: BackupSyncer -> handle synchronization with backup services.

Set up a Marina backup service

The only UI way to set up your backup service is at the onboarding step. onboarding/wallet-restore now provides three choices when selecting the "Ionio restoration":

1. Github -> store the script details in a private Github repo owned by the user (here is mine) (user must set a personnal access token).
2. BrowserSync -> use the Browser.storage.sync storage
3. From file -> former "from JSON file" option

The BrowserSync "problem"

Browser.storage.sync is shared only if the extension is deployed on store. That's why it's hard to test it for now 🤔. Note that it also needs the "extension ID" set up (related to #462).
It's also why I've add the simple Github repo as DB to test it without releasing.

It may need further UI works to close #391 (including a proper settings/backup page). The idea of that PR is to discuss the feature and precise the specs.

please @tiero @bordalix review this