### What changes were proposed in this pull request?

The PR adds a new config option to configure an address for the
proxy server, and a new handler that intercepts redirects and replaces
the URL with one pointing at the proxy server. This is needed on top
of the "proxy base path" support because redirects use full URLs, not
just absolute paths from the server's root.

### Why are the changes needed?

Spark's web UI has support for generating links to paths with a
prefix, to support a proxy server, but those do not apply when
the UI is responding with redirects. In that case, Spark is sending
its own URL back to the client, and if it's behind a dumb proxy
server that doesn't do rewriting (like when using stunnel for HTTPS
support) then the client will see the wrong URL and may fail.

### Does this PR introduce any user-facing change?

Yes. It's a new UI option.

### How was this patch tested?

Tested with added unit test, with Spark behind stunnel, and in a
more complicated app using a different HTTPS proxy.

Closes #26873 from vanzin/SPARK-30240.

Authored-by: Marcelo Vanzin <vanzin@cloudera.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>