[Relay]: Expose resolveOffsetConnection into two functions

Another request!

Is there any way to break the resolveOffsetConnection function into two parts, or change the return type on the `resolveOffsetConnection` callback?

The reason is that I get the `totalCount` and `pageCursors` from one aggregated database call. But I can't make that database call without the offset and limit ... which means I have to manually replicate [the offsetForArgs function](https://github.com/hayes/giraphql/blob/7137675aa4ae69f44793cb384f35ebe61caef514/packages/plugin-relay/src/utils/connections.ts#L21) or roll my own. But then I have two offset/limit calculations which means that if they ever get out of sync, things will get dicey. Plus it just seems inefficient.

I tried returning the totalCount, etc. from the resolveOffsetConnection function but it's expecting an array of nodes, not extra connection info.

Start with packages/plugin-relay/src/utils/connections.ts, especially offsetForArgs, and trace the resolveOffsetConnection callback mentioned in the issue. Determine how the offset and limit can be shared with aggregated connection metadata while preserving the existing node-array behavior; done means callers no longer need to duplicate offset/limit calculation.

Written by the indexing model from the issue text.