Restore encryption support

[simolus3]

This restores support for encrypting databases (removed with the package:sqlite_async update in #390). Encrypting databases does not require additional dependencies anymore. Instead, users enable enable encryption by passing the encryption parameter to a PowerSyncDatabase constructor. This makes the open factory run a pragma cipher = statement to encrypt databases.

For this to work, SQLite3MultipleCiphers must be used. The sqlite3 package supports that, but it requires a configuration option in pubspec.yaml. The EncryptionOptions class documents required steps.

This also adds tests using encrypted databases, both on the web and on native platforms (powersync_sqlcipher was essentially untested).