Issue #3629151: Add a denylist for crawling external URLs

Added the host_denylist configuration option to the External freelinking plugin:

• Added host_denylist textarea to the External plugin configuration form, plugin attribute, and default configuration.
• Added host pattern matching in shouldCrawlUrl() / isHostDenied():
  • Supports exact hostnames and wildcard domain patterns (e.g. *.example.com).
  • Supports host:port patterns and wildcard ports (e.g. *:8080, example.com:8080).
  • Automatically handles user-entered protocol prefixes, trailing slashes, comments (#), and blank lines.
  • Retains existing default protections against crawling localhost and private/reserved IP ranges.
• Added configuration schema in freelinking.schema.yml.
• Added post-update hook freelinking_post_update_external_host_denylist to initialize existing filter format configurations.
• Added unit tests in ExternalHostTest and updated ExternalTest.

Closes #3629151