[WIP] Implement Map ItemReader

[agrare]

The ItemReader looks a lot like a Task state in that it has a "Resource" URI field

https://docs.aws.amazon.com/step-functions/latest/dg/input-output-itemreader.html

Ref: https://states-language.net/#map-state

Reading Items

A Map State MAY have an "ItemReader" field, whose value MUST be a JSON object and is called the ItemReader Configuration. The ItemReader Configuration causes the interpreter to read items from the resource identified by the ItemReader Configuration’s "Resource" field.

The ItemReader Configuration MUST have a "Resource" field, whose value MUST be a URI that uniquely identifies the specific task to execute. The States language does not constrain the URI scheme nor any other part of the URI. The ItemReader Configuration MAY have a "Parameters" field, whose value MUST be a Payload Template. The ItemReader Configuration MAY have a "ReaderConfig" field whose value is a JSON object which MAY have a "MaxItems" field which MUST be a positive integer. The interpreter MAY define additional "ReaderConfig" fields.

The ItemReader Configuration causes the interpreter to read items from the task identified by the ItemReader’s "Resource" field. The interpreter will limit the number of items to the maximum number of items specified by the "ReaderConfig"’s "MaxItems" field. The "MaxItems" can be provided indirectly. A "ReaderConfig" field MAY have "MaxItemsPath" field which MUST be a Reference Path which, when resolved, MUST select a field whose value is a positive integer. A "ReaderConfig" field MUST NOT include both "MaxItems" and "MaxItemsPath".

The default result for "ItemReader" is "$", which is to say the whole effective input.

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