Expose rcl_remap_t in public API

[sloretz]

Feature request

The structure rcl_remap_t should be exposed in the public API, and maybe _rcl_parse_remap_rule() should be to.

Feature description

This is to allow it to be wrapped by rclcpp and passed into the node's constructor as something other than string arguments, which will allow ros2 launch to handle remap rules and parameter files the same way: parse arguments, decide which ones apply to which nodes, and pass just those args into the node's constructor.

Implementation considerations

• remap.h will need to be installed
• rcl_remap_t needs to use PIMPL pattern
• maybe expose _rcl_parse_remap_rule() as a public function to allow creating a remap rule from one string
• rcl_arguments_t needs a method to get remap rules
• Could update rcl_remap_*() functions to accept a list of remap rules instead of an arguments struct since that's all they need.