Signatures in oracle messages updated to Schnorr type (ECDSA signatures specified currently)

[raulcano]

The signatures included in oracle_announcement and oracle_attestation should be specified as Schnorr. However, the way it is described now, it uses the field type signature, which is defined as an ECDSA signature.
This PR creates a new convenience type schnorr_signature and uses it in the descriptions of the oracle_announcement and oracle_attestation.

Current definition of the signature convenience type:

dlcspecs/Messaging.md

Line 126 in 9cd9148

* `signature`: a 64-byte bitcoin Elliptic Curve signature

According to the current spec, the signature within the announcement is a Schnorr signature, but the ECDSA signature field is used. See here an explicit contradition:

dlcspecs/Messaging.md

Lines 418 to 427 in 9cd9148

	#### `oracle_announcement`
	1. type: 55332
	1. data:
	* [`signature`:`annoucement_signature`]
	* [`x_point`:`oracle_public_key`]
	* [`oracle_event`:`oracle_event`]
	where `signature` is a Schnorr signature over a sha256 hash of the serialized `oracle_event`, using the tag `announcement/v0`.

The oracle attestation also needs of Schnorr signatures, but is pointing to the signature type.

dlcspecs/Messaging.md

Lines 436 to 452 in 9cd9148

	#### `oracle_attestation`
	1. type: 55400
	1. data:
	* [`string`:`event_id`]
	* [`x_point`:`oracle_public_key`]
	* [`u16`: `nb_signatures`]
	* [`signature`:`signature_1`]
	* ...
	* [`signature`:`signature_n`]
	* [`string`:`outcome_1`]
	* ...
	* [`string`:`outcome_n`]
	Where the signatures are ordered the same as the nonces in their original `oracle_event`.
	The outcomes should be the message signed, ordered the same as the signatures.

For reference, this implementation of the oracle announcement and attestation uses Schnorr signatures:
https://docs.rs/dlc-messages/latest/dlc_messages/oracle_msgs/struct.OracleAnnouncement.html
https://docs.rs/dlc-messages/latest/dlc_messages/oracle_msgs/struct.OracleAttestation.html