Refactor/Cleanup NetworkRequest and Related Code

[Nerixyz]

Description

This PR does many small changes - each commit can be reviewed on its own.

The following changes were made:

• Cleanup unused code (Twitch v5)
• Add json methods that simplify sending JSON. This also sets the Accepts header, since here, when JSON is sent, only JSON is accepted as a response (this is only done in Helix).
• Helix's makeRequest now accepts an HTTP method.
• Cleaned some clang-tidy suggestions in Network{Request,Private}. The only suggestions left there are (1) a const-cast and (2-5) unused QFuture's from QtConcurrent::run calls. (1) should be resolved when Remove QObjectRef in favor of QPointer #4632 is closed. (2-5) can be resolved by using QThreadPool::start, since we don't need the future.
• Moved asserts to Q_ASSERT_X, which provides better error messages (mainly with a reasoning in the message).

[github-actions]

clang-tidy made some suggestions

[pajlada]

Small nitpicks, otherwise looks good

[Nerixyz]

Constructing the url-query and json in Helix could be simplified by using the constructors taking initializer-lists.

[pajlada]

All changes look good to me 👍
If you want to re-add the assert comments in any way feel free, will merge this in later