[ntcore] add UnitTopic

[spacey-sooty]

This would be a topic that stores a double with unit information represented in NT metadata. This would improve the logging experience for units greatly

[rzblue]

This sounds like a better fit for a higher level telemetry library- we don't want ntcore to depend on wpimath or wpiunits, and the functionality is really just a wrapper around a double topic.

[spacey-sooty]

Maybe something that could be included in wpilibc/j so it's still easily accessible to teams?