RFR: 8349910: Implement JEP 517: HTTP/3 for the HTTP Client API [v16]

[Daniel Fuchs]

> Hi,
> 
> Please find here a PR for the implementation of [JEP 517: HTTP/3 for the HTTP Client API](https://openjdk.org/jeps/517).
> 
> The CSR can be viewed at [JDK-8350588: Implement JEP 517: HTTP/3 for the HTTP Client API](https://bugs.openjdk.org/browse/JDK-8350588)
> 
> This JEP proposes to enhance the HttpClient implementation to support HTTP/3.
> It adds a non-exposed / non-exported internal implementation of the QUIC protocol based on DatagramChannel and the SunJSSE SSLContext provider.

Daniel Fuchs has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 653 commits:

 - merge latest changes from master branch
 - http3: fix at since
 - merge latest http3 changes
 - Remove unused variables
 - Simplify control flow
 - Remove unnecessary instanceof checks
 - Use IntFunction for header generator
 - merge latest changes from master branch
 - http3: minor cleanup
 - http3: cleanup - QuicConnectionImpl::maxInitialTimer should be volatile
 - ... and 643 more: https://git.openjdk.org/jdk/compare/a40afdd0...3925afda

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Changes: https://git.openjdk.org/jdk/pull/24751/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24751&range=15
  Stats: 105850 lines in 474 files changed: 103019 ins; 1333 del; 1498 mod
  Patch: https://git.openjdk.org/jdk/pull/24751.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24751/head:pull/24751

PR: https://git.openjdk.org/jdk/pull/24751