RFR: 8368695: Support 101 switching protocol in jdk.httpserver [v12]

[Josiah Noel]

> Remaking the PR since I messed up the upstream merge on the other one. See (https://github.com/openjdk/jdk/pull/27751) for the bulk of the previous discussion
> 
> - adds a flag to ExchangeImpl to signal whether the current request is a GET Upgrade request
> - Adds a new `UpgradeInputStream`/`UpgradeOutputStream` to ensure that the server keeps track of when the upgraded request is closed
> - on 101 response codes, `sendResponseHeaders` will not immediately close the output stream 
> - on 101 response codes, `sendResponseHeaders` will use an `UpgradeInputStream`

Josiah Noel has updated the pull request incrementally with one additional commit since the last revision:

  constant

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27989/files
  - new: https://git.openjdk.org/jdk/pull/27989/files/cc32d61b..e40b8e1e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27989&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27989&range=10-11

  Stats: 6 lines in 2 files changed: 3 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27989.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27989/head:pull/27989

PR: https://git.openjdk.org/jdk/pull/27989