Don't use fetchJoinCollection on Doctrine Paginator when not needed

[teohhanhui]

Q	A
Bug fix?	yes?
New feature?	no?
BC breaks?	no?
Deprecations?	yes
Tests pass?	yes
Fixed tickets	#2611 (comment), #2611 (comment)
License	MIT
Doc PR	api-platform/docs#771

Refactor QueryChecker etc.

I've removed the flawed handling of "join by class", as we simply have not enough information to do the right thing there. The main caveat (as was before) is that our checking covers the "80%":

• join by association
• no complex expression in SELECT / ORDER BY clauses

but will be confused in the remaining "20%".

For example:

🗹 Supported:

SELECT d, a_1
FROM Dummy d
LEFT JOIN d.relatedDummies a_1
ORDER BY a_1.name

🗷 Not supported:

SELECT d, a_1
FROM Dummy d
LEFT JOIN RelatedDummy a_1 WITH 1 = 1
ORDER BY a_1.name

🗷 Not supported:

SELECT d, LENGTH(a_1.name) AS HIDDEN score
FROM Dummy d
LEFT JOIN d.relatedDummies a_1
ORDER BY score

[teohhanhui]

Since we have no way to cover all use cases, and "we don't know when we don't know" (so we can't even fall back to safe defaults, unless we remove the whole guessing altogether), it's important that we let the user configure these in the resource / operation attributes (done in this PR).

I'd argue that our guessing is worth it, because it'd give the best performance for most users out of the box. I think it's consistent with what I perceive as the philosophy of API Platform, which is to make things effortless for the most common use cases. When the user starts doing more advanced stuff, they are then required to take over control.