prober: add support for custom DNS resolvers

[izumi39]

What this PR does / Which issue(s) does the PR fix:

This PR builds on #1443 to restore and enhance the original custom DNS resolver support.
The original PR has been open for a long time and appears to be no longer maintained.
I have based this PR on that branch and made the following updates:

1. Resolve merge conflicts from the original PR to make it mergeable.
2. Add documentation for the new dns_server and dns_timeout module configuration options, including example usage in the configuration docs.
3. Set a sensible default for dns_timeout to 3 seconds when not explicitly configured, to avoid zero-value timeout behavior.

Fixes #1443

Does this PR introduce a user-facing change?

[FEATURE] Add support for custom DNS resolver with `dns_server` and `dns_timeout` options. `dns_timeout` defaults to 3s if unset

Checklist

• Tests updated
• Documentation added
• CHANGELOG added in release-notes section of PR Desc.