fix: handle item iri with identifiers in LegacyIriConverter

[jonnyeom]

Q	A
Branch?	2.7
Tickets	#5620
License	MIT

Hello, a small fix for when using the \ApiPlatform\Core\Api\LegacyIriConverter

The Problem:

#5630 Introduced a nice way to fix the IriConverterInterface deprecation before turning off the metadata_backward_compatibility_layer.

One issue I ran into is that LegacyIriConverter::getIriFromResource decides if the iri will be Item level or Collection level solely based on if the given $item parameter is a string vs an object.

With this methodology, I would have instantiate a new object just to trigger the Item level iri conversion.
(To compare, The new IriConverterInterface handles this decision smartly, using the metadata of the Operation and the given Object).

Proposed solution

Previously, the old IriConverterInterface had 2 distinct methods to split this (getItemIriFromResourceClass and getIriFromResourceClass)— the only difference being an extra parameter array $identifiers.

This adds back the ability to identify a Resource based on className + identifiers.

[dunglas]

Nitpicking, otherwise LGTM, but I'll wait for @soyuka review before merging because he knows this part better!

[soyuka]

lgtm