[bazel_ros2_rules] Allow specifying the RMW implementation at the WORKSPACE level

[sloretz]

The goal of this ticket is to allow specifying the RMW implementation for a project in only one place. Currently the only way to specify the RMW implementation is to do so on every ros_* rule.

Should work when using a local repository

ros2_local_repository(
    name = "ros2",
    rmw_implementation="rmw_cyclonedds_cpp",
    ...
)

Or when using an archive:

ros2_archive(
    name = "ros2",
    rmw_implementation="rmw_cyclonedds_cpp",
    ...
)

Context: #278

---

One way to implement this would be:

• Add rmw_implementation to base_ros2_repository_attrs
• Pass the value of that rmw implementation into the distro.bzl file generation tool as a CLI argument
• Pass in the value as a new template variable called DEFAULT_RMW_IMPLEMENTATION, else pass in the empty string when generating the template
• Use that value in distro.bzl.tpl with a set-if-not-set action for the environment variable RMW_IMPLEMENTATION

I'm pretty sure this would allow ros_cc_binary(rmw_implementation="..." to override the default, but would need to test to actually be sure.