Add support for constants in messages

[nnmm]

Currently, constants (such as string MY_CONSTANT_STRING="ros2_rust") are not supported yet by rosidl_generator_rs. It would be nice to support them, for feature parity with other languages.

Should they be const or static on the Rust side?

[Nizerlak]

Should they be const or static on the Rust side?

Is there any reason, why they should be static instead of const?

[nnmm]

Should they be const or static on the Rust side?

Is there any reason, why they should be static instead of const?

Some of the constants have types that cannot be constructed as a const, e.g. string types, but with a static we could use the lazy initialization workaround (e. g. using https://crates.io/crates/once_cell).

[nnmm]

Done in #269