Add a specific return type / type constraint to `Keyfunc` and `SignedString`

[Christian Banse]

Currently, Keyfunc expects to return an interface{} and SignedString expects a interface{} parameter. This often lead to confusing, what exactly these functions expect. We should therefore constrain the types of these functions. A possible solution could look like this:

diff --git a/token.go b/token.go
index c8ad7c7..644b15e 100644
--- a/token.go
+++ b/token.go
@@ -1,15 +1,24 @@
 package jwt

 import (
+	"crypto"
 	"encoding/base64"
 	"encoding/json"
 )

+type ParsingKey interface {
+	crypto.PublicKey | []uint8 | unsafeNoneMagicConstant
+}
+
+type SigningKey interface {
+	crypto.PrivateKey | []uint8 | unsafeNoneMagicConstant
+}
+
 // Keyfunc will be used by the Parse methods as a callback function to supply
 // the key for verification.  The function receives the parsed, but unverified
 // Token.  This allows you to use properties in the Header of the token (such as
 // `kid`) to identify which key to use.
-type Keyfunc func(*Token) (interface{}, error)
+type Keyfunc func(*Token) (ParsingKey, error)

 // Token represents a JWT Token.  Different fields will be used depending on
 // whether you're creating or parsing/verifying a token.
@@ -46,7 +55,7 @@ func NewWithClaims(method SigningMethod, claims Claims, opts ...TokenOption) *To
 // https://golang-jwt.github.io/jwt/usage/signing_methods/#signing-methods-and-key-types
 // for an overview of the different signing methods and their respective key
 // types.
-func (t *Token) SignedString(key interface{}) (string, error) {
+func (t *Token) SignedString(key SigningKey) (string, error) {
 	sstr, err := t.SigningString()
 	if err != nil {
 		return "", err