[cmd3] Add Commands v3-compatible SydIdRoutine

[DeltaDizzy]

Because SysIdRoutine is contained within the Commands v2 library, an equivalent is needed for Commands v3 to enable SysId usage.

Feedback sought on child command names.

[DeltaDizzy]

Additional questions:

1. Should SysIdRoutine's Mechanism inner class be distinguished from Commands v3's Mechanism by a rename or namespace qualification? Currently I have it renamed to SysIdMechanism but am not sure.
2. Should the removal of null parameters in favor of defaults and use of static Config factories be ported to the Commands v2 version of SysIdRoutine?

[Gold856]

We don't do m_ for public fields. At least, that's the general convention across WPILib (it's not super formalized). Separately, we should clean it up in the v2 SysIdRoutine.

[DeltaDizzy]

We don't do m_ for public fields. At least, that's the general convention across WPILib (it's not super formalized). Separately, we should clean it up in the v2 SysIdRoutine.

Cleanup of v2 is planned for a followup PR because I wanted to keep this PR focused, but if it's not too much of a scope creep I can do it here I suppose.

[SamCarlberg]

This class should have tests. The v2 SysIdRoutine tests are a good starting point to copy over.