RFR: 8368955: Improve HttpExchange.sendResponseHeaders usability

[Josiah Noel]

As specified in [JDK-8368955](https://bugs.openjdk.org/browse/JDK-8368955?jql=project%20in%20(JDK)%20AND%20component%20in%20(core-libs)%20AND%20Subcomponent%20in%20(java.net)), adds some constants to HttpExchange to make `sendResponseHeaders` more usable.

-------------

Commit messages:
 - Improve HttpExchange.sendResponseHeaders usability

Changes: https://git.openjdk.org/jdk/pull/27613/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27613&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368955
  Stats: 20 lines in 1 file changed: 12 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/27613.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27613/head:pull/27613

PR: https://git.openjdk.org/jdk/pull/27613