Implement 'preconnect-https' and 'preconnect-http' for H2

QNetworkAccessManager::connectToHostEncrypted()/connectToHost()
creates 'fake' requests with pseudo-schemes 'preconnect-https'/
'preconnect-http'. QHttp2ProtocolHandler should handle this
requests in a special way - reporting them immediately as
finished (so that QNAM emits finished as it does in case of
HTTP/1.1) and not trying to send anything.
We also have to properly cache the connection - 'https' or
'http' scheme is too generic - it allows (unfortunately)
mixing H2/HTTP/1.1 in a single connection in case an attribute
was missing on a request, which is wrong.
h2c is more complicated, since it needs a real request
to negotiate the protocol switch to H2, with the current
QNetworkHttpConnection(Channel)'s design it's not possible
without large changes (aka regressions and new bugs introduced).

Auto-test extended.

Fixes: QTBUG-77082
Change-Id: I03467673a620c89784c2d36521020dc9d08aced7
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

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