[protobuf][wpimath] Move serde functions to separate classes

[pjreiniger]

Prep work for #5888

The protobuf and struct de/serialization tooling is ripe for auto generation. Moving the utility classes to their own files will make that a whole lot easier.

In addition, this adds units to the field names in the protobuf messages. Like java, the generated protobuf files have no concept of what unit they represent (not even with comments at the moment). I've updated all of the fields that use some unit in C++. Having the unit type in the name can also assist with the generation process.

I did not update the struct schema with unit names. I don't know if there is some magic where that has to map to the protobuf name or if it is just for display purposes

[PeterJohnson]

If we're going to embed the unit type in the field names, we should do it in both struct and proto for consistency. In both cases it will show up as the field's name in our introspective tools like OutlineViewer and AdvantageScope. I actually considered using the protobuf extensions method to add a custom "units" option, but it didn't play nicely with QuickBuffers (the Java protobuf implementation we're using).

[PeterJohnson]

Thinking about this, organizationally I think it would be better to have separate "struct" and "proto" directories/packages, rather than a unified "serde" package. I also don't see a reason for the class names to have Serde appended, they're already in a package and uniquely named with Struct and Proto.

[PeterJohnson]

For consistency and to reduce potential confusion, let's name the .h files in the proto/ and struct/ directories with Proto and Struct suffixes the same way as the Java files are.

[pjreiniger]

/format

[PeterJohnson]

Please break the units change into a separate PR (we'll merge the change to separate classes first). I think there's probably more discussion/changes to be had re: adding units, such as documenting that the units of X and Y are meters on the Java side.