RFR: 8368695: Support 101 switching protocol in jdk.httpserver

[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`

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Commit messages:
 - Update SwitchingProtocolTest.java
 - Create UpgradeInputStream.java
 - Delete UpgradeInputStream.java
 - Support 101 upgrade requests

Changes: https://git.openjdk.org/jdk/pull/27751/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27751&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368695
  Stats: 278 lines in 3 files changed: 270 ins; 2 del; 6 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