RFR: 8329829: HttpClient: Add a BodyPublishers.ofFileChannel method

[Volkan Yazici]

Adds a new `ofFileChannel(FileChannel channel, long offset, long length)` method to `java.net.HttpRequest.BodyPublishers` to provide an `HttpClient` publisher to upload a certain region of a file. The new publisher does not modify the state of the passed `FileChannel`, streams the file channel bytes as it publishes (i.e., avoids reading the entire file into the memory), and can be leveraged to implement sliced uploads. As noted in the Javadoc:

> The file channel will not be closed upon completion. The caller is
> expected to manage the life cycle of the channel, and close it
> appropriately when not needed anymore.

### Implementation notes

- `FileChannel` is preferred over `{Readable,Seekable}ByteChannel`, since the latter does not provide a positional read without modifying the state of the `FileChannel`, which is necessary to use a single `FileChannel` instance to implement sliced uploads.
- `ofFileChannel(FileChannel,long,long)` is preferred over `ofPath(Path,long,long)` to avoid overloading the maximum file descriptor limit of the platform.

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Commit messages:
 - Add `HttpRequest.BodyPublishers::ofFileChannel` along with tests

Changes: https://git.openjdk.org/jdk/pull/26155/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26155&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329829
  Stats: 836 lines in 4 files changed: 831 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26155.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26155/head:pull/26155

PR: https://git.openjdk.org/jdk/pull/26155