Add SwiftPM release packaging + workflow

[reez]

Description

Adds a public SwiftPM entry at the repo root using a binary target that points to a release asset zip, this will let folks not have to use a local script if they are wanting to use swift bindings, they can just point to this repo using Swift Package Manager.

Notes to the reviewers

This is a bit different way to do this from bdk-ffi, with this we dont need to create a separate rust-ctktap-swift repo or anything.

Also of note, preferred way to do a rust-cktap release now that will cut the release and also create+tag a Swift Package is to run the “Release Swift Package” workflow with a tag like v0.1.1 (or 0.1.1), which will build the xcframework, zip it, update Package.swift, commit the Swift sources, tags, and upload the asset.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[reez]

@notmandatory I'll merge, do a test-release on my fork, then cut a real release here this month