feat(bdk_wallet): add helper method to get the KeyMap for Psbt.sign

[oleonardolima]

Describe the enhancement

It might be useful to use the helper method in order to get the Keymap for the existing Wallet, to be used with the new way of signing with Psbt.sign.

Use case

Impact

• Blocking production usage
• Nice-to-have / UX improvement
• Developer experience / maintainability

Are you using BDK in a production project?

• Yes
• No
• Not yet, but planning to

Which backend(s) are relevant (if any)?

• Electrum
• Esplora
• Bitcoin Core RPC
• None / not backend-related (e.g. bdk_chain, bdk_core)
• Other (please specify): ____

Project or organization (optional)

Additional context

[ValuedMammal]

How about something like this? (noting that it's still blocked by rust-bitcoin/rust-miniscript#867).

impl Wallet {
    /// Sign with descriptor.
    pub fn sign_psbt_with_descriptor<D>(
        &self,
        psbt: &mut Psbt,
        descriptor: D,
    ) -> Result<SigningKeysMap, (SigningKeysMap, SigningErrors)>
    where
        D: IntoWalletDescriptor,
    {
        let (_, keymap) = descriptor
            .into_wallet_descriptor(&self.secp, self.network)
            .unwrap();
        let keymap = KeyMapWrapper::from(keymap);
        psbt.sign(&keymap, &self.secp)
    }

    /// Sign with key.
    pub fn sign_psbt<K>(
        &self,
        psbt: &mut Psbt,
        key: &K,
    ) -> Result<SigningKeysMap, (SigningKeysMap, SigningErrors)>
    where
        K: GetKey,
    {
        psbt.sign(key, &self.secp)
    }
}