Add support for BIP-129 (BSMS)

**Describe the enhancement**

The [BIP-129] spec proposes a mechanism for Bitcoin Secure Multisig Setup (BSMS). This issue is to determine if/how BDK can support BSMS. It's also possible that this functionality could be split into a separate crate for use outside of BDK.

**Use case**

Many users of BDK use it to create and manage multisig wallets and need to set them up securely. By implementing BSMS functionality we could provide a secure and interoperable way for setting up multisig wallets with BDK.

**Additional context**

This enhancement was suggested by @NVK.

[BIP-129]: https://github.com/bitcoin/bips/blob/master/bip-0129.mediawiki