Feature Proposal: BIP322 message signing

**Describe the enhancement**
https://github.com/bitcoin/bips/blob/master/bip-0322.mediawiki

**Use case**

In switzerland it's possible to offer exchange services (buy/sell) up to ~900€/d without extended kyc (they only get the IBAN obviously). These services are available to all EU citizens (SEPA net) and one of the prefered ways of bitcoiners in the EU to stack sats (besides p2p).
These service need to (by law) verify the ownership of the address of the user (trough message signing). They mostly either build their own apps (eg Relai) which do it in the background, or let the user do it manually with their preferred wallet (eg Pocket Bitcoin). It would probably be much easier for the exchanges to build good wallets with simple ux with bdk if it supported message signing. Also wallets built on BDK could support manual message signing so users can use the wallet to take custody/buy on these exchanges.

**Additional context**
There are maybe other relevant usecases of message signing i'm not aware of. More use cases listed here: https://hackmd.io/KR5ALrSSQO6eyrnUBJreeA