Supporting algorithms from extra processing provider plugins

[florisvdh]

@JanCaha, this is a follow-up of your request in r-spatial/qgisprocess#122 (comment).

This is to remind that r-spatial/qgisprocess#122 has been merged with support for enabling/disabling QGIS processing provider plugins, and an upcoming tweak is that qgis_providers() will also return the count of algorithms per provider.

The qgis package could maximize the availability of extra functions from processing provider plugins, by offering them in the package. Maybe the package could also take advantage of below internal function after loading qgisprocess at startup, in order to signal disabled plugins.

> qgisprocess:::message_disabled_plugins(qgisprocess::qgis_plugins())
>>> Run `qgis_enable_plugins()` to enable 1 disabled plugin(s) and
    access their algorithms: cartography_tools