RenderMan boilerplate

[johnhaddon]

As mentioned in the previous user group, I've been working on RenderMan support for a little while. I'm sitting on quite a lot of unmerged code at this point, and will need to get the work so far reviewed and merged while I continue to add missing features. Rather than one huge unreviewable code-dump, I'm aiming to break this up into a number of smaller, more digestible PRs, of which this is the first.

This just includes some really minimal boilerplate to start things off gently :

• An IECoreRenderMan library with a single ParamListAlgo header, for translating IECore data to RenderMan's RtParamLists.
• A CI setup for downloading RenderMan and building against it on GitHub Actions. As for the Arnold setup, this is only operational for release builds and pull requests made from GafferHQ/gaffer, not forks.

[murraystevenson]

It begins! 🫖

This all looks fine to me from a does it download RenderMan and build something standpoint. Nice work on figuring out the correct powershell incantations. Looking forward to the next lot! 😁