Add Cesium3DTileset.pickPrimitive

[ebogo1]

• Adds option to pass a primitive to Cesium3DTileset constructor for rendering primitives during the pick pass.

[cesium-concierge]

Thanks for the pull request @ebogo1!

• ✔️ Signed CLA found.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

[ebogo1]

@lilleyse Should be ready after 7d46b8b.

[lilleyse]

@ebogo1 I made a few updates right at the end related to the doc and tests.

• Changed @type {Primitive} to @type {Object} since in this context a primitive is any object that conforms to the primitive interface (which isn't super well defined, but it's an object that has an update function, a show property, and other common properties. For example, a 3D Tileset is considered a primitive. The Primitive type is specific to boxes, spheres, and other geometries.)
• Simplified the test, mostly to ensure that all tiles are loaded after maximumScreenSpaceError is set