[META] Factor away the RequestHandler class

Added issue links to summary

So far, the combined result of this is:

 jsonapi.services.yml                                 |   8 ++--
 src/Controller/EntityResource.php                    | 205 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++--------------------------
 src/Controller/EntryPoint.php                        |   2 +-
 src/Controller/RequestHandler.php                    | 187 ++++++----------------------------------------------------------------------
 src/LinkManager/LinkManager.php                      |   2 +-
 src/Normalizer/Value/RelationshipNormalizerValue.php |   2 +-
 src/Routing/Routes.php                               |  40 +++++++++++------
 tests/src/Functional/ConfigurableLanguageTest.php    |   2 +-
 tests/src/Functional/ExternalNormalizersTest.php     |   2 +-
 tests/src/Functional/JsonApiFunctionalTest.php       |  10 ++---
 tests/src/Functional/JsonApiRegressionTest.php       |   2 +-
 tests/src/Functional/MessageTest.php                 |   4 +-
 tests/src/Functional/NodeTest.php                    |   4 +-
 tests/src/Functional/ResourceResponseTestTrait.php   |   3 +-
 tests/src/Functional/ResourceTestBase.php            |  18 ++++----
 tests/src/Functional/TermTest.php                    |   4 +-
 tests/src/Functional/UserTest.php                    |   6 +--
 tests/src/Kernel/Controller/EntityResourceTest.php   | 434 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--------------------------------------------------------------------------------------------------------------------------
 18 files changed, 350 insertions(+), 585 deletions(-)

That does NOT include "Remove RequestHandler class and service and add EntityResource methods to each route definition" which will almost certainly be a large net reduction.

Woaahhhh! Where is this coming from all of a sudden?! I thought we were gonna focus on the bug reports. I guess you got annoyed by it one time too many? :)

(In any case, thanks for splitting up!)

Precisely. This has been on my bucket list for months. But it was blocked by the render context we had in the RequestHandler, which you moved to an event subscriber, and the complexity (perhaps perceived) of Routes.php, which we already refactored.

TBH, this was actually pretty low-effort because it's really just pure refactoring. I spent a few hours over the weekend doing this.. consider it therapeutic refactoring :P

I think the purpose here is to decrease complexity.

@Wim Leers. That's definitely my intention!

"Complexity" is probably a bit in the eye of the beholder though, so @e0ipso, I see your point of view. You said that this could "[scatter] a front controller into services that are not explicitly invoked".

You're right that deserialization and parameter resolution will happen before to the controller is called and that's a little magical. But I don't know if I would say it's not explicit. It'll all be declared on the route definition.

In some ways, it'll become more explicit. I.e. the method that actually handles each operation wont be called dynamically through call_user_function_array and the class that handles deserialization could be called something very explicit, like IncomingEntityDeserializer.

I've removed #2987606: Remove config mutation tests from EntityResourceTest from the list of sub-issues. It doesn't need to block this progress. It was just easier to remove the config tests than it was to fiddle with them to make them work again. I ended up doing that anyway in #2987608-8: Move deserialization from RequestHandler to JsonApiParamEnhancer, that means we can just move on with the rest of this issue and remove those tests independently.

Fixing some issue summary formatting.

This let's us start to clean up EntityResource.

Adding #2991841: Remove unused parameters from EntityResource methods.

#2987609: Rename the entity parameter from the entity type ID to 'entity' for all routes landed.

Formatting.

#2991841: Remove unused parameters from EntityResource methods. landed.

this. is. DONE.