Add Swift sync stream helpers

[simolus3]

This:

• Hides the default subscribe method. Because it takes value classes, it's tricky to safely call from Swift.
• Adds a swift-specific subscribe method that takes primitive types instead.
• Adds a helper for SyncStatusData.forStream that doesn't require a StreamDescription, so that we don't have to implement that interface in Swift.

This unblocks: powersync-ja/powersync-swift#86