Support ReturnValuesOnConditionCheckFailure

[bartelink]

For ConditionCheckFailedException, its now possible to have the exception include the content of the conflicting item in the same roundtrip, by setting ReturnValuesOnConditionCheckFailure flag in the request)

The library should either turn that on unconditionally (there is no RU charge impact, though obviously the response will be larger)

• For Equinox, the APIs that it's needed for are TransactWriteItems, PutItemAsync and UpdateItemAsync https://github.com/jet/equinox/blob/8d44e38eeccbc3bf9ccd1d28892d14a50a36a66a/src/Equinox.DynamoStore/DynamoStore.fs#L575-L577
• But, really, any API that's annotated as potentially triggering a Precondition.CheckFailed, the option should be available to include it in the response

(NOTE the AWS SDK's exception type has long been plumbed for holding the content in question - the new bit is the fact that a DDB server will now include it in the response for single Item APIs as TransactWriteItems has done for some time).

It's unclear to me whether a SDK update will be necessary (my money is on no though)

[njlr]

On this, we could extend the active pattern, e.g.

/// Helpers for identifying Failed Precondition check outcomes emanating from <c>PutItem</c>, <c>UpdateItem</c> or <c>DeleteItem</c>
module Precondition =
    /// <summary>Exception filter to identify whether an individual (non-transactional) <c>PutItem</c>, <c>UpdateItem</c> or <c>DeleteItem</c> call's <c>precondition</c> check failing.</summary>
    let (|CheckFailed|_|): exn -> unit option =
        function
        | :? ConditionalCheckFailedException -> Some()
        | _ -> None

    let (|CheckFailedWithItem|_|) (template : RecordTemplate<'TRecord>) (exn : exn) : ('TRecord voption) option =
        match exn with
        | :? ConditionalCheckFailedException as ccfe ->
            if ccfe.Item <> null then
                let item = template.OfAttributeValues ccfe.Item
                Some (ValueSome item)
            else
                Some ValueNone
        | _ -> None

But I wonder if it would be better to make the item part of the return type rather than matching exceptions?

This way the library can offer some type system guard rails.
We can also turn off ReturnValuesOnConditionCheckFailure where a precondition is not given.

Just a sketch:

type PreconditionCheckFailed<'TRecord> =
  | PreconditionCheckFailed of 'TRecord

TryPutItem : 'TRecord -> Async<TableKey>

TryPutItemWithPrecondition : 'TRecord * precondition : ConditionExpression<'TRecord> -> Async<Result<TableKey, PreconditionCheckFailed<'TRecord>>