Add support for sObject Collections

[yukas]

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[yukas]

@spectator At this point I only needed the create/update functionality for RP. There is also retrieve/delete functionality which sObject collections API provides. I can implement those too. Should I do it right away in this PR, or we can merge this one and let it be used on LP and I implement retrieve/delete in a separate PR?

[spectator]

@spectator At this point I only needed the create/update functionality for RP. There is also retrieve/delete functionality which sObject collections API provides. I can implement those too. Should I do it right away in this PR, or we can merge this one and let it be used on LP and I implement retrieve/delete in a separate PR?

Since we don't need retrieve/delete right now, let's finish create/update and RP functionality first, and then we'll discuss. Please submit PR to gem maintainers to add create/update when you have a moment.