Teach Visual Editor about multiple entry points

[dglazkov]

Now that we have multiple entry points (as a result of #2736), we need to figure out how to expose them in Visual Editor. What's the right UX? @paullewis showed me a cool mock once with a little "Start" button next to the component. Maybe it's like that, with the ability to drag it around? Or what? I have no idea -- yet!

Interesting tidbits about entry points (see some docs in https://breadboard-ai.github.io/breadboard/docs/reference/runtime-semantics/#entry-points):

• A graph may have zero or more entry points.
• If there aren't any defined, components with no incoming wires will be used as entry points.
• Entry points have labels. Currently, we only have default and describe, with invoke (see #2793 ) coming soon.
• We can imagine creating more types of labels, like subgraph, which allows us to build subgraphs right next to the main graph and then invoke them

See a demo of a describer entry point: https://youtu.be/vseteCOUhhU

[dglazkov]

@donsbach obi wan