[ApolloPagination] Support queued operations

[Iron-Ham]

This is an experimental change which aims to add support for queued operations.
I'm testing this branch in a sample application here.

This change attempts to address the need for secondary triggers in SwiftUI projects, relevant to my comment here: #317 (comment)

For more information, read the README of my sample under the Secondary Triggers known issue.

This pull request adds support for queued operations. Each queued operation is setup such that only one queued operation per set of current data can be executed. If, for example, I call loadNext rapidly three times while nothing is actively loading, the first call will trigger a pagination load, as before. The second call will queue to be executed after the completion of the first call, but will throw a loadInProgress error (as before). The third call will behave as it did before: by only throwing the error. Once the first call is complete, the second pagination fetch will execute.

I am using the current data state to disambiguate these fetches. That does mean that 3 calls to loadNext and 3 calls to loadPrevious will queue both a loadPrevious and a loadNext.

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This pull request primarily focuses on improving the functionality of the AsyncGraphQLQueryPagerCoordinator in the apollo-ios-pagination package. The changes include the introduction of an Operation enum and a queuedOperations property to handle and queue operations, and modifications to the loadNext, loadPrevious, and reset methods to execute these queued operations. Additionally, the paginationFetch method has been updated to handle pagination errors and queue operations accordingly.

Here are the most important changes:

• apollo-ios-pagination/Sources/ApolloPagination/AsyncGraphQLQueryPagerCoordinator.swift: An Operation enum has been introduced within the AsyncGraphQLQueryPagerCoordinator actor that includes two cases: loadNext and loadPrevious. Each case can hold a CachePolicy and a PaginatedQuery.

• apollo-ios-pagination/Sources/ApolloPagination/AsyncGraphQLQueryPagerCoordinator.swift: A new property queuedOperations of type OrderedSet<Operation> has been added to store the operations to be executed.

• apollo-ios-pagination/Sources/ApolloPagination/AsyncGraphQLQueryPagerCoordinator.swift: The loadPrevious and loadNext methods now include a call to executeQueuedOperations after the paginationFetch call. This ensures that any queued operations are executed after a page load operation. [1] [2]

• apollo-ios-pagination/Sources/ApolloPagination/AsyncGraphQLQueryPagerCoordinator.swift: The reset method now clears all queued operations.

• apollo-ios-pagination/Sources/ApolloPagination/AsyncGraphQLQueryPagerCoordinator.swift: The paginationFetch method has been updated to queue operations in case of a loadInProgress or missingInitialPage error. A new private method _paginationFetch has been introduced to handle the actual fetch operation.

• apollo-ios-pagination/Sources/ApolloPagination/AsyncGraphQLQueryPagerCoordinator.swift: A new method executeQueuedOperations has been introduced to execute all operations in the queue sequentially and then clear the queue.

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