Add support for experimental Cli configuration

Allow to mark some commands and flags experimental on cli (i.e. not
depending to the state of the daemon). This will allow more flexibility
on experimentation with the cli.

Marking `docker trust` as cli experimental as it is documented so.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

e2e/internal/fixtures/fixtures.go

@@ -32,7 +32,8 @@ func SetupConfigFile(t *testing.T) fs.Dir {
 			"https://notary-server:4443": {
 				"auth": "ZWlhaXM6cGFzc3dvcmQK"
 			}
-		}
+		},
+		"experimental": "enabled"
 	}
 	`))
 	return *dir