feat: support loading Elixir modules for auth

[marc0s]

Allow to specify an Elixir module name in auth_method. This would allow to create custom auth modules using Elixir.

If the referenced module, M, cannot be loaded as ejabberd_auth_M, try to load it as Elixir.M.

This is my first contribution to this repo (and my first in Erlang at all), so please excuse me any basic mistake I could have done. Tell me and I'll fix them :)

[marc0s]

This can be tested with this dummy module https://gist.github.com/marc0s/754bf23e3a4a01ab831246342b1e7621

Tell me if you prefer it to be included in the PR to be added to the sources (like mod_presence_demo.ex, for instance)

[marc0s]

Is there something I can do about the failing CI checks?

Seems that some files are missing and that they're failing for other PRs...

[badlop]

Great PR, thanks!

Is there something I can do about the failing CI checks?

All the failed checks are produced by just one single error, reported by the Dialyzer static analysis tool. That small patch should solve it . Please apply to your code and resubmit:

diff --git a/src/econf.erl b/src/econf.erl
index f94d0db77..0ffd05e4e 100644
--- a/src/econf.erl
+++ b/src/econf.erl
@@ -511,7 +511,7 @@ db_type(M) ->
             ElixirModule = "Elixir." ++ atom_to_list(T),
             case code:ensure_loaded(list_to_atom(ElixirModule)) of
               {module, _} -> list_to_atom(ElixirModule);
-              {error, x} -> fail({bad_db_type, M, T})
+              {error, _} -> fail({bad_db_type, M, T})
             end
           end
       end).

Tell me if you prefer it to be included in the PR to be added to the sources (like mod_presence_demo.ex, for instance)

Yes, can you include it with this name lib/ejabberd_auth_example.ex? Don't worry if it doesn't work correctly, I'm planning a few improvements in that and other elixir files, will commit them after merging your PR.

[badlop]

Now that Dialyzer succeeds, the test progress and now the Elvis style reviewer complains:

# src/ejabberd.erl [FAIL]
  - no_space (https://github.com/inaka/elvis_core/tree/main/doc_rules/elvis_style/no_space.md)
    - Unexpected space to the left of "," on line 185

It refers to that line:

<<"Elixir" , _/binary>> ->  misc:binary_to_atom(hd(T));

Right now it has a space, for example <<"one" , "two">> and it is preferable to write <<"one", "two">>

[coveralls]

coverage: 32.91% (-0.002%) from 32.912%
when pulling 17b5b34 on Quobis:support-loading-elixir-modules-for-auth
into c291c20 on processone:master.