invoices: migrate KV invoices to native SQL for users of KV SQL backends

[bhandras]

Change Description

This pull request adds the migration of old key-value (KV) invoices to the new native SQL schema when the --db.use-native-sql flag is set, unless the --db.skip-sql-invoice-migration flag is also specified.

Please note that since we currently do not support running on mixed database backends for users of bbolt or etcd, an additional step is required to migrate their KV database to SQL first. For more context, please see https://github.com/lightninglabs/lndinit/pull/21.