Action message support

[nwn]

This implements action support in rosidl_generator_rs and rosidl_runtime_rs. It's a selective rebase of the original changes made in #295 and #410, applying only changes to the two rosidl packages, without any modification to rclrs. The intention here is to merge these changes sooner so that we can split the message support packages into a separate repo and add them to the ROS buildfarm.

The general approach follows the same pattern as the existing message and service generation. However, the additional message wrapping and introspection done on actions means that we need generic access to certain message internals. In rclcpp, this is done using duck typing on the Action and Action::Impl template parameters. However, Rust is stricter about this and requires trait bounds to ensure that generic type parameters can be accessed in a given way. As a result, a series of trait methods are defined in ActionImpl that enable a client library like rclrs to generically create and access the various message types.

Certain of these methods involve accessing timestamps and UUIDs in messages. To avoid adding a dependency in rosidl_runtime_rs on builtin_interfaces and unique_identifier_msgs, these are represented in a more primitive form as (i32, u32) and [u8; 16] in the signatures, respectively. This is rather ugly, so ideas are welcome.

The existing srv.rs.em file is split into separate "idiomatic" and "RMW" implementations, similarly to how messages are already handled. This makes embedding the service template into the action.rs.em easier.