Also, made mod_private_backend module

Move new code into mongoose_backend, just to keep it away from the old backend_module
Align args correctly

mod_private without dynamic modules

Add `mod_auth_token_backend`

Co-authored-by: Paweł Chrząszcz <pawel.chrzaszcz@erlang-solutions.com>

It returns the atom describing the backend, like `mssql`, `pgsql`, etc.
This is analogous to `backend_name` function that was exposed by the
dynamically compiled modules in `backend_module.erl`. It is used by
some tests and `mongoose_rdbms`.

Because backends can be running per host type. This also fixes an
issue of suites logging that they finished dirty, after having
dynamically compiled backends removed.

The old interface is called for the time being, but will be removed when
all modules will have dynamic backends converted.

Add `mod_event_pusher_push_backend`

Add mongoose_backend:get_backend_name

Migrate mod_privacy to mongoose_backend

mod_inbox

Note also the many dialyzer error that were hiding behind a dynamic
callstack. They were all wrong specs and not wrong code, but still,
nice not to hide them any longer.

Add `mod_muc_backend`

Migrate mod_muc_light_db to mongoose_backend

Wdbm/vcard

Removing dynamically compiled module from mod_inbox

Remove mod_inbox_backend from xref_ignores

…e-dynamic-modules

Add `mod_pubsub_cache_backend`

Looks good, thanks for the fix

Add host types into mod_mam_cassandra_arch

Use db_message_format=mam_message_xml by default now in mod_mam_muc_cassandra_arch and mod_mam_cassandra_arch
Get rid of mod_mam_muc_cassandra_arch_params

Remove dynamically compiled modules from mod_http_upload

Use persistent terms to store backend_module in ejabberd_sm

…-dynamic-modules

…ynamic-modules

…erd-router

With the changes done in #3386, this turned out to be really easy. Before that, I tried to make mongoose_rdbms_backend expose its API with HostTypes, but it was a bit messy. I think it's better to do it in a separate PR (after the whole without-dynamic-backend-modules branch is merged), to keep this already big diff smaller and more focused.

Other issue is that mongoose_rdbms has many functions exported, which seem to be not used, mainly for escaping different types. This could be revisited, but, in similar fashion, I left this as is for now.
What's left to do is remove dynamic mongoose_rdbms_type, but it will be done in a separate PR.

Get rid of ejabberd_rdbms because all it did was compile mongoose_rdbms_type. In turn, all it did was get an option from the config.

The functionality is left as it was. From what I understand, one can (in theory) use ODBC with different databases, so maybe the rdbms_server_type option is needed, but it could at least be moved from the general section to the RDBMS pool options. This could be done after dynamically compiled modules are merged.