refactor!: remove DataLoaderPreparedRequestExecutor

[stuebingerb]

Adds proper handling for dataProperties to ParallelRequestExecutor and gets rid of DataLoaderPreparedRequestExecutor, along with the option to configure the request executor in SchemaConfiguration.

This also consolidates some tests that were hardcoded to use one of the request executors, and therefore missed the other.

Resolves #58
Resolves #268

BREAKING CHANGE: The option to configure request executors has been removed, and execution will now always use the ParallelRequestExecutor.

[stuebingerb]

This feels way too easy but there seems to be only one test that actually validates DataLoader specifics (DataLoaderTest#data loader cache per request only).

Already open for review but I want to have a bit more test coverage.

Edit: basic data loader validates as well but I still feel like I'm missing something.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.26%. Comparing base (3a71dba) to head (9d37255).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #305      +/-   ##
==========================================
+ Coverage   73.23%   74.26%   +1.02%     
==========================================
  Files         161      159       -2     
  Lines        5142     4923     -219     
  Branches      910      847      -63     
==========================================
- Hits         3766     3656     -110     
+ Misses       1035      955      -80     
+ Partials      341      312      -29     

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[stuebingerb]

Removing the one request executor that already works with kotlinx.serialization seems bad for #224 but my gut feeling is that this would still be a step in the right direction. Both executors have diverged and I believe it is easier to migrate the existing one than solving existing issues with DataLoaderPreparedRequestExecutor and adding missing functionality there. Feel free to disagree, though, happy to have a discussion.