DDC-3622: [GH-1336] Fix UoW warning with custom id object types

[doctrinebot]

Jira issue originally created by user @doctrinebot:

This issue is created automatically through a Github pull request on behalf of stefanotorresi:

Url: #1336

Message:

UnitOfWork::tryGetById() and UnitOfWork::tryGetByIdHash() may receive an object as an id, for instance when using a custom id type like Rhumsaa\Uuid.

This PR adds a simple cast to string in these methods.

There are a couple of things I'm not sure about, tho.

First of all, I don't know if functional tests alone are deemed sufficient, but I couldn't figure out how to replicate the issue via unit tests only (apart from directly invoking the two methods above with an object as argument, which looks a bit pointless).

Furthermore, I don't know if the cast should be applied elsewhere; so far the only cases I stumbled upon where common find and fetch joins, but I saw other methods where it may be necessary (i.e. here and here), but I couldn't replicate the code path to reach those.
Also I wonder if an exception should be thrown if the id object doesn't implement **toString(), and in this case if the fix should be refactored into Doctrine\ORM\Utility\IdentifierFlattener, for a bit of SoC.

Let me know what you think.

[doctrinebot]

Comment created by @doctrinebot:

A related Github Pull-Request [GH-1336] was closed:
#1336

[doctrinebot]

Issue was closed with resolution "Fixed"