Add SQLite3 backup

[n8maninger]

Adds an endpoint to backup a running hostd node to the local system. Can be used to schedule automatic atomic backups without shutting down the node or locking the database for a significant amount of time.

[POST] /api/system/sqlite3/backup

curl -u :"sia is cool" http://localhost:9980/api/system/sqlite3/backup -X POST -d '{ "path": "/backups/hostd.sqlite3.bak" }'

[ChrisSchinnerl]

Overall this looks good. Only thing I don't love is passing paths to a foreign filesystem through an endpoint but I guess that can't be avoided if we want to allow performing a backup through the UI.