Simplify `AppCrypto` offchain type conversion

[rakanalh]

As a follow up to the refactoring of #5182, AppCrypto trait type conversions need some simplification. The scription attached explains what needs to be done.

substrate/frame/system/src/offchain.rs

Lines 397 to 438 in 4a6d268

	/// App-specific crypto trait that provides sign/verify abilities to offchain workers.
	///
	/// Implementations of this trait should specify the app-specific public/signature types.
	/// This is merely a wrapper around an existing `RuntimeAppPublic` type, but with
	/// extra non-application-specific crypto type that is being wrapped (e.g. `sr25519`, `ed25519`).
	/// This is needed to later on convert into runtime-specific `Public` key, which might support
	/// multiple different crypto.
	/// The point of this trait is to be able to easily convert between `RuntimeAppPublic` and
	/// the wrapped crypto types.
	///
	/// TODO [#???] Potentially use `IsWrappedBy` types, or find some other way to make it easy to
	/// obtain unwrapped crypto (and wrap it back).
	///
	/// Example (pseudo-)implementation:
	/// ```
	/// // im-online specific crypto
	/// type RuntimeAppPublic = ImOnline(sr25519::Public);
	/// // wrapped "raw" crypto
	/// type GenericPublic = sr25519::Public;
	/// type GenericSignature = sr25519::Signature;
	///
	/// // runtime-specific public key
	/// type Public = MultiSigner: From<sr25519::Public>;
	/// type Signature = MulitSignature: From<sr25519::Signature>;
	/// ```
	pub trait AppCrypto<Public, Signature> {
	/// A application-specific crypto.
	type RuntimeAppPublic: RuntimeAppPublic;
	/// A raw crypto public key wrapped by `RuntimeAppPublic`.
	type GenericPublic:
	From<Self::RuntimeAppPublic>
	+ Into<Self::RuntimeAppPublic>
	+ TryFrom<Public>
	+ Into<Public>;
	/// A matching raw crypto `Signature` type.
	type GenericSignature:
	From<<Self::RuntimeAppPublic as RuntimeAppPublic>::Signature>
	+ Into<<Self::RuntimeAppPublic as RuntimeAppPublic>::Signature>
	+ TryFrom<Signature>
	+ Into<Signature>;

[tomusdrw]

AppKey looks like a good candidate, it's a trait that links all the app crypto types and wrapped types.