examples: add a Log4j 2 logging context example

[AgraVator]

Adds example-log4j2, showing how a server interceptor can put per-call values into the Log4j 2 ThreadContext so they end up on every log line the service writes. Logging frameworks read from thread-local storage rather than io.grpc.Context, so the interceptor has to set the values on whichever thread runs each callback.

2026/09/14 09:10:54:450 UTC INFO  CustomLogServer - Server started, listening on 50051
2026/09/14 09:11:02:679 UTC INFO  CustomLogServer - Got a request {requestId=4da3d1d9-47b6-46b7-876d-8af1b16f65dd, clientName=my.domain.name}

This revives #5835 by @carl-mastrangelo, which was reviewed but never merged; the example is his. It picks up the review comments that were left open there: the directory is named example-log4j2 so the Kokoro loop finds it, the BUILD.bazel copied from the ALTS example and the vendored gradle wrapper are gone, the README says what the example is for and names the right scripts, and the versions are current.

It also sets the context around next.startCall(), not just in the listener callbacks, so interceptors further down the chain see it too. That was the gap @Nipel-Crumple raised on #5835.

The server uses LogManager and a log4j2.xml with %X in the pattern instead of building a SimpleLogger by hand, since that's what a reader would actually copy.