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 <[email protected]>

* Authority discoverty signing error revisited

* Signing error revisited for babe and aura as well

* Further cleanup

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Co-authored-by: Koute <[email protected]>