Add `Smith2018ArticularContactForce` and `Smith2018ContactMesh` as extension components

[adamkewley]

Reported by @sabrinahoermann

She's making/editing a model that's related to this repository from @clnsmith:

• https://github.com/clnsmith/opensim-jam

And that, internally, uses jam-plugin (in binary form), from here:

• https://github.com/opensim-jam-org/jam-plugin

Which isn't loadable in OpenSim Creator, because OSC is built from source on Windows/Mac/Linux from a not-necessarily-release version of OpenSim (binary plugins typically are only guaranteed to work with a specific version of OpenSim).

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The proposed fix for this is to add the source code for the jam plugins directly into OpenSim Creator as "extension" or "non-standard" OpenSim components. This will mean that any .osim files in the wild that happen to use those components will work in OpenSim Creator without having to support a plugin architecture.

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The reason why OpenSim Creator doesn't support plugin loading (and, imo, shouldn't) is because OpenSim isn't necessarily ABI-stable over time (only API-stable). It's very hard to maintain a binary plugin architecture across multiple platforms, architectures, etc. - even modern language platforms resort to shipping source code that's compiled on the target machine (which would be hard for us, because I really don't want to ship a C++ compiler alongside OSC).

If users desire a long-term plugin architecture then I would recommend someone develops one in (e.g.) python, so that application builders don't have to worry about porting every plugin to every native architecture.

[adamkewley]

02f1e946 is related to this issue (forgot to tag it)

[adamkewley]

Verified with @sabrinahoermann that this is now capable of editing the relevant models. I'll add some basic tests to ensure they stay in and then close this.