st0601: add support for multiple Control Command entries

[bradh]

Resolves #140

Motivation and Context

The Control Command pack can be repeated in a single ST 0601 local set. We don't currently support that, and will overwrite any earlier entries with the last encountered entry. This is currently tracked as #140.

Description

Adds a "holder" class to capture each entry. Its not really clean because of how we serialise out the entries as shown in https://github.com/WestRidgeSystems/jmisb/blob/develop/api/src/main/java/org/jmisb/api/klv/st0601/UasDatalinkMessage.java#L138-L145

I've introduced a special "marker" interface to handle the special case. Currently this is the only class that implements it, but there are more coming (we just don't implement that functionality yet). There are other ways to do this, but this seemed a way to avoid changing existing code everywhere.

How Has This Been Tested?

Unit testing only.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.