Add WITH_PROTOBUF CMake flag for disabling protobuf serialization

[calcmogul]

SleipnirGroup/Choreo#727 is using wpimath's spline trajectory generator as an initial guess for the optimization problem. Building has proven difficult because it requires protobuf installed in CI. We don't need serialization, so we wrote a Python script to manually patch it out. However, upstream keeps breaking it in nontrivial ways.

An upstream CMake flag like WITH_PROTOBUF that disables protobuf would make our lives easier.

[PeterJohnson]

Duplicate of #5887 re: protobuf disabling. Struct has no external dependencies and it should not be necessary to disable it.