Qt Network Authorization module improvements

A container Epic for improving qtnetworkauth module. Tentative high level prioritization below.

First (6.8?)

• [Needs new API] From new feature point of view, introducing convenience for using private URI schemes and https uri schemes is probably the most important.
  • QTBUG-124338
  • QTBUG-124339
  • QTBUG-124340
• [Needs new API] From security point of view, PCKE convenience should be introduced.
  • QTBUG-124327
• From existing code point of view, we should improve documentation
  • QTBUG-124323
  • QTBUG-124324
  • QTBUG-124325
• From existing code point of view, we should address few high priority bugs (eg. double-encoding which prevents certain vendors)
  • QTBUG-81624
  • QTBUG-85265

Then (6.9?, or if time permits, selectively in 6.8)

• [Needs new API] Improve OpenID Connect support
  • QTBUG-124336
  • QTBUG-124334
• [Needs new API] New convenience (token expiration, accepted scopes)
  • QTBUG-124330
  • QTBUG-124332
• Address lower priority bugs and issues in the existing code

And then
Going even further, we should investigate in-app browser tab usage.

• QTBUG-124328