Encryption support

[simolus3]

With the latest CSQLite and Kotlin SDK release, we can finally support encryption in the Swift SDK as well. On the SDK, the only change is that we add a parameter controlling which statements to run when databases are opened. This allows running a pragma key = to set encryption keys.

Actually using encryption is a bit trickier:

• The CSQLite package exports encryption support as a SwiftPM package trait.
• So ideally our Swift SDK would have the same trait and then depend on CSQLite, forwarding the trait. This is not supported by SwiftPM though.
• And even if it were supported, XCode doesn't support setting package traits on dependencies.

As a workaround, users have to:

• Add a local Package.swift setup somewhere in their project sources.
• In that project, depend on CSQLite with the encryption trait.
• Then depend on that local project from XCode. Since the CSQLite package can only appear once in the entire dependency graph, this makes XCode enable the trait.
• Add an initializing statement to configure encryption.

This also adds a new Demo project following those steps.

[stevensJourney]

This looks good to me. I left some comments for some minor items I think may be outstanding.

[stevensJourney]

Happy with the changes :)