[ldd-proc] Add software availability attribute to `proc:Software`

[cgobat]

Summary/Rationale It would be useful to have an additional attribute under the <proc:Software> class that allows data providers to specify the public availability status of processing software. It could be called something along the lines of software_availability, and would be a sibling of attributes such as software_id, software_version_id, etc.

For example, when software/pipelines are proprietary and/or pre-date SPD-41a, such an attribute would potentially prevent data users from wasting their time trying to search for a piece of software that was never released and that they will never be able to find. On the flip side, open source/publicly available software should also be recognized and acknowledged as such. Some ideas for potential possible values for this attribute:

• Unreleased
• Proprietary
• Open source
• Public / Available (e.g. if binary has been released, but not source code)
• ...?

PDS4 IM Version 1.18.0.0 (1I00), at least

Need-by Date Not particularly urgent, but sometime in the next several months would be nice—I may submit a PR myself that adds this.

Additional context @katecrombie can provide more info/details.

[cgobat]

The add-software-availability branch of my fork that is now linked in the "Development" section of this issue contains a proposal for a possible implementation of this. See draft PR pds-data-dictionaries/ldd-proc#26.

[cgobat]

Resolved by pds-data-dictionaries/ldd-proc#29. Thank you @rsjoyner and @jordanpadams for your work on this!