decouple service accounts from root credentials (#14534)

changing root credentials makes service accounts
in-operable, this PR changes the way sessionToken
is generated for service accounts.

It changes service account behavior to generate
sessionToken claims from its own secret instead
of using global root credential.

Existing credentials will be supported by
falling back to verify using root credential.

fixes #14530

cmd/jwt_test.go

@@ -149,7 +149,7 @@ func TestWebRequestAuthenticate(t *testing.T) {
 	}
 
 	for i, testCase := range testCases {
-		_, _, _, gotErr := webRequestAuthenticate(testCase.req)
+		_, _, _, gotErr := metricsRequestAuthenticate(testCase.req)
 		if testCase.expectedErr != gotErr {
 			t.Errorf("Test %d, expected err %s, got %s", i+1, testCase.expectedErr, gotErr)
 		}