refactor: rename token introspection token_type to token_use

BREAKING CHANGE: Previously, the OAuth2 Token Introspection endpoint would return `access_token` or `refresh_token` for the key `token_type`. This however is not according to spec, which specifies `bearer` as the only valid (unless an extension is used) `token_type` parameter. Please be aware that `token_type_hint` in the **request parameters** is still correct. For more information consult [RFC7662](https://tools.ietf.org/html/rfc7662). If you wish to know if a token can be used as an access or refresh token, check the new `token_use` parameter!

Closes #1762

oauth2/handler.go

@@ -437,7 +437,7 @@ func (h *Handler) IntrospectHandler(w http.ResponseWriter, r *http.Request, _ ht
 	resp := &fosite.IntrospectionResponse{
 		Active:          true,
 		AccessRequester: ar,
-		TokenType:       tt,
+		TokenUse:        tt,
 	}
 
 	exp := resp.GetAccessRequester().GetSession().GetExpiresAt(tt)
@@ -475,7 +475,7 @@ func (h *Handler) IntrospectHandler(w http.ResponseWriter, r *http.Request, _ ht
 		Audience:          resp.GetAccessRequester().GetGrantedAudience(),
 		Issuer:            strings.TrimRight(h.c.IssuerURL().String(), "/") + "/",
 		ObfuscatedSubject: obfuscated,
-		TokenType:         string(resp.GetTokenType()),
+		TokenType:         string(resp.GetTokenUse()),
 	}); err != nil {
 		x.LogError(r, errors.WithStack(err), h.r.Logger())
 	}

oauth2/introspector.go

@@ -78,9 +78,12 @@ type Introspection struct {
 	// IssuerURL is a string representing the issuer of this token
 	Issuer string `json:"iss,omitempty"`
 
-	// TokenType is the introspected token's type, for example `access_token` or `refresh_token`.
+	// TokenType is the type of the token. It will always be `bearer`. Do not confuse this with TokenUse!
 	TokenType string `json:"token_type,omitempty"`
 
+	// TokenUse is the introspected token's use - is one of `access_token` or `refresh_token`.
+	TokenUse string `json:"token_use,omitempty"`
+
 	// Extra is arbitrary data set by the session.
 	Extra map[string]interface{} `json:"ext,omitempty"`
 }