RFR: 8245095: Implementation of JEP 408: Simple Web Server

[Julia Boes]

This change implements a simple web server that can be run on the command-line with `java -m jdk.httpserver`.

This is facilitated by adding an entry point for the `jdk.httpserver` module, an implementation class whose main method is run when the above command is executed. This is the first such module entry point in the JDK.

The server is a minimal HTTP server that serves the static files of a given directory, similar to existing alternatives on other platforms and convenient for testing, development, and debugging.

Additionally, a small API is introduced for programmatic creation and customization.

Testing: tier1-3.

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Commit messages:
 - fix whitespace
 - Move changes from sandbox to mainline

Changes: https://git.openjdk.java.net/jdk/pull/5505/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5505&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245095
  Stats: 6891 lines in 42 files changed: 6856 ins; 15 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5505.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5505/head:pull/5505

PR: https://git.openjdk.java.net/jdk/pull/5505