Generalize auth/types.StdSignature (#4507)

New Signature interface available in the top level types package.
auth.StdSignature implements such interface. User defined auth
module can now define their own custom signature types.

Work carried out in the context of the following issues:
- #4488
- #4487

docs/spec/auth/03_types.md

@@ -20,9 +20,9 @@ type StdFee struct {
 
 ## StdSignature
 
-A `StdSignature` is the combination of an optional public key and a cryptographic signature
-as a byte array. The SDK is agnostic to particular key or signature formats and supports any
-supported by the `PubKey` interface.
+`StdSignature` implements the `sdk.Signature` interface and consists of a combination of an
+optional public key and a cryptographic signature as a byte slice. The SDK is agnostic to
+particular key or signature formats and supports any supported by the `PubKey` interface.
 
 ```golang
 type StdSignature struct {