C++ SysIdRoutine Mechanism does not support null log function

[bhall-ctre]

Describe the bug
In the Java version of SysIdRoutine.Mechanism, you can pass in null for the log parameter in the constructor. However, the C++ version is not a std::optional, and passing in nullptr or {} causes a crash.

To Reproduce
Steps to reproduce the behavior:

1. Create a C++ SysIdRoutine where the log parameter for Mechanism is nullptr or {}
2. Try running any SysIdRoutine command

Expected behavior
To match the behavior in Java, and following the convention of the Config class, the C++ Mechanism log parameter should be a std::optional, and passing in std::nullopt should not cause a crash. Alternatively, the Config class should be modified not to take a std::optional<std::function> recordState, and both should just accept a (potentially null) std::function.

The temporary workaround is to pass in an empty single-parameter lambda: [](auto) {}.