Add support for list `Socket`s

[nickbianco]

Partially fixes issue #3605

Brief summary of changes

This PR adds support for list Sockets via the new macro OpenSim_DECLARE_LIST_SOCKET. This addition is motivated by current work to support Controllers in Moco. The need for this change is explained in #3605.

In addition to the macro, Component and Socket has new getConnectee() signatures that accept an index for list socket support. Most of the remaining changes are implementation details (largely based on how list Inputs are constructed).

Testing I've completed

Updated a few tests in testComponentInterface.cpp to test list Sockets. These tests include serialization/deserialization.

Looking for feedback on...

Some of the checks on the connectee paths could probably be simplified (see the TODOs).

CHANGELOG.md (choose one)

• updated.

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This change is 

[adamkewley]

Ah, I wasn't aware that Input had that behavior: it's not an API I've used!

[nickbianco]

After comparing to Property, I can see how it can be a bit confusing. Part of the current design might be because Sockets and Inputs add an internally managed property to handle serialization and deserialization. Although, I think we could probably bring these APIs closer together (in a future PR, of course 😄).

[adamkewley]

I'm unsure what's best, it isn't an API I have used (yet).

I'd vote for designing it to be logical, rather than backwards-compatible, in this case. I can't imagine that there are many downstream implementors of the virtual API (connect) outside of this repository, and I can't imagine that many codebases are using the Input API much (yet). E.g. even the Output API (arguably, easier to integrate) is mostly only used by OSC right now?

[nickbianco]

@adamkewley, @aymanhab ready for review.

[nickbianco]

@adamkewley good to merge?

[nickbianco]

Thanks @adamkewley and @aymanhab for the detailed reviews! This one was tricky.