Closed by unintentional language in another PR comment - still working on this

I am actively working on this right now (for any interested parties). I've gathered more information to illustrate how the RMW API is currently incomplete.

The following values are gathered from get_publishers_info_by_topic when a single publisher is publishing with unspecified Deadline duration. It doesn't matter which RMW implementation is being used to publish, the value only depends on the implementation of the subscripton - I have tested it with only Cyclone and FastDDS, with all 4 pub/sub combinations. These same values are reported for Deadline, Lifespan, and Liveliness Lease Duration

"DDS IDL PSM Constant" above refers to the values laid out in Section 2.3.3 of the DDS 1.4 spec. From my understanding this PSM is meant to provide common interfaces for programming languages. There are separate docs available as well:

• C language (Cyclone) - https://www.omg.org/spec/DDS/20140501/dds_dcps.idl
• C++ language (FastDDS) - https://www.omg.org/spec/DDS-PSM-Cxx/1.0/PDF
  The C++ one doesn't lay out new constants, from what I understand is it is meant to replace the IDL-generated API with more idiomatic language

Based on the above information, the two implementations are providing values that are neither consistent with each other or guidelines from the specification - seems like we just have exposed implementation details.

My next step is an updated API proposal, which will come with coupled prototype PRs for the RMW implementations.

Interesting note: the IDL PSM specifies Durations as:

struct Duration_t {
  long sec;
  unsigned long nanosec;
};

But we use exclusively unsigned values within the RMW API.

Closing this in favor of #301 for now