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

[Josiah Noel]

> - adds a flag to ExchangeImpl to signal whether the current request is an Upgrade request
> - Adds a new `UpgradeInputStream` 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 `UndefLengthOutputStream`

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

  reduce diff

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Changes:
  - all: https://git.openjdk.org/jdk/pull/27751/files
  - new: https://git.openjdk.org/jdk/pull/27751/files/a3250508..8872bca5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27751&range=11
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27751&range=10-11

  Stats: 12 lines in 1 file changed: 5 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/27751.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27751/head:pull/27751

PR: https://git.openjdk.org/jdk/pull/27751