[glass, wpilib] Rewrite Mechanism2d (#3281)

Substantially improves Mechanism2d by moving it to NetworkTables and adding
a robot API to create the mechanism elements, instead of requiring a JSON file.

Co-authored-by: Peter Johnson <johnson.peter@gmail.com>

wpilibcExamples/src/main/cpp/examples/examples.json

@@ -51,6 +51,14 @@
     "gradlebase": "cpp",
     "commandversion": 2
   },
+  {
+    "name":"Mechanism2d",
+    "foldername":"Mechanism2d",
+    "gradlebase":"cpp",
+    "description":"An example usage of Mechanism2d to display mechanism states on a dashboard.",
+    "tags":[],
+    "commandversion": 2
+  },
   {
     "name": "Solenoids",
     "description": "Demonstrate controlling a single and double solenoid from Joystick buttons.",