Support opening remote databases

[t-h-e]

Follow up to https://github.com/keepassxreboot/keepassxc/pull/7222

Adding an option to import a remote database. Import the data to a new database, existing database or a temporary database (see option "Temporary Database" in section "Import Into")

Screenshots

Testing strategy

Similar to syncing with a remote database, specify a download command and if necessary input to download the remote database.

Also added a unit test.

Type of change

• ✅ New feature (change that adds functionality)