[Core, mod_commands] Interface allowlist (#3086)

* [core] Add interface allowlist to gate module app/api registration

Adds an optional, presence-activated allowlist in switch.conf.xml that
controls which modules may register application / api / json_api /
chat-application interfaces. With no <interface-allowlist> configured
nothing is enforced; when at least one <allow> entry is present, only
listed interfaces register at load time and all others are refused (the
module still loads and switch_loadable_module_process still returns
SUCCESS -- the blocked interface is simply never exposed).

Entries match at three levels of precision:
  mod_commands            - whole module
  mod_commands.system     - any interface named "system"
  mod_commands.system.api - a specific type (app|api|json_api|chat_app)

Enforcement lives in switch_loadable_module_process() so every module, at
boot and at runtime `load`, is subject to the same policy. This gives
operators a way to disable the "system"/"spawn" shell-exec API commands
(and equivalents) system-wide.

Also adds the `interface_allowlist_dump [modules] [plain]` API, which
walks the loaded modules and prints their interfaces in the allowlist key
format so the current state can be captured and pruned offline into config.

* [mod_commands] Add tests for the interface allowlist

New test_interface_allowlist boots the core with an active
<interface-allowlist> (conf_interface_allowlist/) that permits only a
couple of mod_commands interfaces, then loads mod_commands and verifies:

- listed commands register and run (status, version) while unlisted and
  shell-exec commands are refused (system, spawn, uptime) -- refusal
  surfaces as switch_api_execute returning FALSE / command-not-found,
  with the command function never invoked;
- a "module.name.type" entry gates by type: the API "status" loads while
  the JSON API of the same name stays blocked;
- interface_allowlist_dump prints the config format in its xml, modules
  and plain variants, and reflects module capabilities (system appears in
  the dump even though it was blocked from registering).

* [config] Fix interior -- in interface-allowlist comment breaking XML parse

The explanatory comment used -- as em-dash pairs. The XML parser treats
-- inside a comment as the comment close, causing an "unclosed <!--"
error that prevents the whole freeswitch.xml from parsing (boot and
reloadxml both fail). Replace the -- pairs with ordinary punctuation.

* update .gitignore

* [core] Warn when interface-allowlist section is present but parses no entries

Co-authored-by: Chris Rienzo <chris@signalwire.com>

.gitignore

@@ -282,5 +282,6 @@ images/test_text.png
 
 src/mod/codecs/mod_amrwb/test/test_amrwb
 src/mod/endpoints/mod_sofia/test/sipp-based-tests
+src/mod/applications/mod_commands/test/test_interface_allowlist
 
 .DS_Store