New uniffi release related to swift

[reez]

Describe the enhancement

new uniffi release has some swift specific stuff that might make, making the xcframeworks easier: https://mozilla.github.io/uniffi-rs/next/swift/uniffi-bindgen-swift.html

[reez]

I've been doing more experimenting with this locally, but first step for the repos is to update to uniffi 0.28.2 for both:

• bitcoin-ffi chore: uniffi 0.28.2 bitcoin-ffi#28
• bdk-ffi (draft) chore: uniffi 0.28.2 #614