[ApolloPagination] Additional conveniences for offset pagination

[Iron-Ham]

While writing docs in #262, I recognized that support for offset pagination could be better. Specifically:

• We could support bidirectional and reverse pagination
• We could have additional convenience methods for that pagination type

As such, this pull request modifies the OffsetPagination class to be a namespace, just like CursorBasedPagination. We introduce three new structs to facilitate pagination:

• OffsetPagination.Forward: Introduced OffsetPagination.Forward structure to represent a forward pagination state. This is identical to the previous contents of OffsetPagination.
• OffsetPagination.Reverse: Introduced OffsetPagination.Reverse structure to represent a reverse pagination state.
• OffsetPagination.Bidirectional: Introduced OffsetPagination.Bidirectional structure to represent a bidirectional pagination state.

With the addition of these new structures, we could now add and update convenience make functions for offset pagination, which add support for Bidirectional pagination – as well as add additional transform options, supporting both single-model output and Collection output.

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cc: @AnthonyMDev I'm glad that you pointed out the need for offset pagination docs. I've focused almost entirely on cursors, as that's what we use at GitHub. Once this is merged, I'll look into:

• Updating the docs to accommodate for these changes
• Introducing a test suite around offsets, as I realize that our existing test suite is similarly focused on cursors.
• I could additionally implement convenience functions for multi-query pagination using offsets, but I'm a bit hesitant. The number of convenience make functions is quite large, and I'm concerned that it would be confusing for users to parse through them. Yes, they could filter by typing: make followed by their direction Forward, and their pagination type Offset or Cursor, which will then present them with a further six possibilities: (single-query, single-query collection transform, single-query single-model transform, multi-query, multi-query collection transform, multi-query single-model transform). Perhaps for now, it's best to add it and then re-think the way that convenience initializers are exposed. Perhaps we can do something silly and make use of namespaces? Imagine a case-less CursorPager and OffsetPager, with further namespaces for direction (Forward, Reverse, and Bidirectional, that then had the specific convenience functions available? This can be an improvement for later.

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[AnthonyMDev]

Approved with the caveat that we are going to remove a bunch of the convenience make functions as we discussed privately.

[Iron-Ham]

Closing in favor of #268