[Feature request]: Support the ability to describe runtime-variable constraints in nav graphs

[mxgrey]

There may be cases where robots can or cannot use certain nav graph elements (lanes or vertices) depending on runtime variables, such as what size payload they are carrying. We should provide users a way to express such constraints in the site editor.

One question to consider is how to express the constraints. Here are some initial options I can think of:

• a free-form text editor that supports a JSON format
• a key-value pair editor that lets users add rows with new key-value pairs
• a plugin-based system that allows downstream users to add more types of constraints to support