add starlark debug server address flag

[1seal]

Summary

• add --experimental_skylark_debug_server_address while preserving the existing default bind behavior
• bind the Starlark debug server to an explicit address only when the new flag is set
• cover both the default wildcard bind and an explicit loopback bind with focused unit tests

Why

This introduces the configuration knob first without breaking existing cross-machine debugger workflows. Users and internal tools can opt into a specific address now, and callers that need remote debugger access can explicitly set a non-loopback address before any future default change.

User impact

there is no behavior change by default.
users who want local-only debugger access can pass --experimental_skylark_debug_server_address=localhost or another loopback address.
users who intentionally attach from another host can continue relying on the current default for now, or set the required listen address explicitly.

Validation

• git diff --check
• /tmp/bazelisk test --test_output=errors //src/test/java/com/google/devtools/build/lib/starlarkdebug/server:StarlarkDebugServerTest //src/test/java/com/google/devtools/build/lib/starlarkdebug/server:StarlarkDebugIntegrationTest

[hvadehra]

While this is a reasonable change, it is a breaking one. That should be fine for Bazel, since this will go live only with Bazel 10. But will almost certainly end up being an issue internally where that will happen in O(days), and I strongly suspect tools / services have come to rely on this working across machines.

What I would suggest is we first introduce the flag with the default being the old/existing behavior. We can then update things internally to prepare for changing the default in a followup change (hopefully in just a week or two).

[hvadehra]

The docs for InetAddress.getName say the method already behaves exactly this way for null or empty inputs, so this is unnecessary.