Keystore overhaul (final) (#13683)

* Introduce keystore specialized sign methods

* Get rid of 'AppKey::UntypedGeneric' associated type.

Untyped generics are accessible using associated types 'Generic' associated type.
I.e. <T as AppKey>::Public::Generic

* Get rid of 'CryptoTypePublicPair'

* Trivial fix

* Small refactory of local keystore implementations

* Remove 'crypto_id' method from 'Public'

* Trivial rename of 'AppKey' to 'AppCrypto'

* Remove unused import

* Improve docs

* Better signature related errors for authority-discovery

* Apply review suggestion

* Apply review suggestions

Co-authored-by: Koute <koute@users.noreply.github.com>

* Authority discoverty signing error revisited

* Signing error revisited for babe and aura as well

* Further cleanup

---------

Co-authored-by: Koute <koute@users.noreply.github.com>

substrate/client/authority-discovery/src/error.rs

@@ -18,8 +18,6 @@
 
 //! Authority discovery errors.
 
-use sp_core::crypto::CryptoTypePublicPair;
-
 /// AuthorityDiscovery Result.
 pub type Result<T> = std::result::Result<T, Error>;
 
@@ -59,11 +57,8 @@ pub enum Error {
 	#[error("Failed to parse a libp2p key.")]
 	ParsingLibp2pIdentity(#[from] libp2p::identity::error::DecodingError),
 
-	#[error("Failed to sign using a specific public key.")]
-	MissingSignature(CryptoTypePublicPair),
-
-	#[error("Failed to sign using all public keys.")]
-	Signing,
+	#[error("Failed to sign: {0}.")]
+	CannotSign(String),
 
 	#[error("Failed to register Prometheus metric.")]
 	Prometheus(#[from] prometheus_endpoint::PrometheusError),