ConcatenateListMergeFunction should return lazy iterables

[t92549]

Describe the new feature you'd like
The federated store's ConcatenateListMergeFunction should return results in a lazy iterable.

Why do you want this feature?
Pre v2.0 the results were in a lazy iterable so this functionality should be retained.

Additional context
Currently, the koryphe function ToList is used to turn all results into iterables. However, this stops the results from being returned lazily as they are read into a list:

Gaffer/store-implementation/federated-store/src/main/java/uk/gov/gchq/gaffer/federatedstore/util/ConcatenateListMergeFunction.java

Line 36 in 2a33be8

final Iterable<Object> updateSafe = isNull(update) ? Collections.emptyList() : (Iterable<Object>) new ToList().apply(update);

As well as this, Lists.newArrayList has the same issue:

Gaffer/store-implementation/federated-store/src/main/java/uk/gov/gchq/gaffer/federatedstore/util/ConcatenateListMergeFunction.java

Line 39 in 2a33be8

: new IterableConcat<>().apply(Lists.newArrayList(updateSafe, state));

Koryphe functions do not really need to be used at all here in fact, and IterableConcat can be replaced by directly using a ChainedIterable, which is what used to happen and what is happening indirectly anyway.

If the proposed changes were implemented, then gchq/koryphe#286 can be closed.

As well as these functionality changes, there should be proper javadoc and tests added to this class to ensure it works as expected and the expected behaviour is documented.

Additionally, I would argue the class should be renamed to ConcatenateMergeFunction and ConcatenateSetMergeFunction should be deleted.
This is because results are concatenated whether they are in a list or not, and returned in an Iterable. The results are not necessarily returned in a list. If a user specifically wanted a list or set return type, they could use ToList or ToSet in their operation chain.

[GCHQDev404]

ChainedIterable requires an Array of Iterables to be given to the constructor.

You don't know what is being returned from the individual graphs, until it's returned and they may not be Iterables.

3 graphs could return:
3 nulls
3 iterates of elements
3 schemas
3 strings
3 lists of strings.

Also you need to have all the iterates available as the same time which doesn't work the same way as an BiFunction being looped and updated.

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edit

are you talking about change this line new IterableConcat<>().apply(Lists.newArrayList(updateSafe, state)); to ChainedIterable ?

[t92549]

I understand you need to be flexible, but the way it used to be done retained lazy iterables, whereas now it does not:

Gaffer/store-implementation/federated-store/src/main/java/uk/gov/gchq/gaffer/federatedstore/operation/handler/impl/FederatedOperationChainHandler.java

Line 88 in 9421d9d

return new ChainedIterable(CollectionUtil.toIterableArray((List) results));

[t92549]

Closed by #2831