Add methods to NodeAPI for signmessage and checkmessage [issue #109]

[lnd3v]

Early (partial) commit.

This commit augments the Breez SDK with message signing and verification facilities. Changes span across the Node API interface, its implementation (Greenlight), BreezServices and the UniFFI binding, with amendments in the FFI definition.

• Added sign_message() and check_message() methods to the NodeAPI trait and Greenlight.
• Added async calls for these methods in BreezServices.
• Exposed said calls via the UniFFI binding for FFI.
• Updated the FFI definition to mirror these additions.

Todo:

[ x] formatting
[ x] tests
[ ] etc.

[roeierez]

Thank you @lnd3v looks good, dropped some comments.

[lnd3v]

For the record, i had to ask about signing on the greenlight discord. The response I got:

cdecker — Today at 15:50
We are currently using the signmessage method to create attestations and authorizations for keyless clients. If we were to allow clients to call signmessage they could escalate privileges trivially.

We are going to change that soon and signmessage will be enabled. In the meantime you may find the vls code helpful in deriving the secret and signing the message.

After that it was easily to find the signing code. It works when doing a copy and paste, however when trying to use the vls code directly it fails as trait fields are private. If they were public it would be as simple as:

# cargo.toml
vls-core = "0.9.1-rc.2"
# node_api.rs
use lightning_signer::signer::derive::{KeyDerive, NativeKeyDerive};
async fn sign_message(&self, message: &str) -> Result<String> {
    let native_key_derive = NativeKeyDerive { network: bitcoin::network::constants::Network::Bitcoin };
    let (_pubkey_from_node_id, seckey) = KeyDerive::node_keys(&native_key_derive, &self.secret, &Secp256k1::new());
    println!("_pubkey_from_node_id: {} ", hex::encode(_pubkey_from_node_id.serialize()));
    Ok(sign(message.as_bytes(), &seckey).unwrap())
}

However since the NativeKeyDerive trait doesn't have a public constructor i'll go ahead and do the copy & paste approach for now, which will just have to get cleaned out once the signer's sign_message works as expected for signing lightning messages.

[JssDWt]

Some small comments about cleaning up.

[JssDWt]

• Rebased
• Upgraded greenlight dependency to include the merged sign_message PR
• Fixed react native bindings. Android works, iOS needs to be tested still.

[JssDWt]

Renamed signed_message to signature

[JssDWt]

Added sign/check message to the flutter bridge
@erdemyerebasmaz can you check whether I did that right?

[JssDWt]

• rebased
• Updated swift mappings in sdk-react-native. iOS now also works.

[dangeross]

Looks good! Left 2 nitty comments

[erdemyerebasmaz]

Added sign/check message to the flutter bridge @erdemyerebasmaz can you check whether I did that right?

@JssDWt looks good!

[JssDWt]

PR is ready to merge.
Is it a problem that the greenlight dependency is updated in this PR @ok300 @roeierez?

[ok300]

Generally LGTM, left a few comments.

I don't think it's a problem that the GL dependencies were updated, especially since the new GL version is needed for this feature.

[ok300]

Fixes #109

[JssDWt]

• Updated documentation on functions and structs.
• Rebased