tutor plugins enable xxx does not update the environment

[regisb]

Bug description

Enabling a plugin should properly update the environment.

How to reproduce

When running:

tutor plugins enable cairn

I would expect that the environment is updated, as per this new feature. But it turns out it's not the case. The following command does not output anything:

grep cairn ~/.local/share/tutor/env/local/docker-compose.yml

Additional context

This is a caching issue. The ENV_PATCHES_DICT variable is a cache that is filled once all plugins are loaded -- but not when a new plugin is loaded. This is an issue that was discussed before here: #917 (comment)

Turns out, cache management is even more difficult that we anticipated. This issue also shows up in the MFE plugin, which has to manually implement, cache management, which is ugly: https://github.com/overhangio/tutor-mfe/blob/de526369f0a882c459ac9d20575f8c23ea0f7fab/tutormfe/plugin.py#L93

Still, we do want to preserve caching features for performance reasons. So here's what I suggest:

1. Create a hooks.Actions.PLUGINS_CACHE_CLEAR action.
2. Create a new tutor/hooks/utils.py module, with Apache 2.0 license.
3. In that new module, create a @plugins_cache decorator that will be a wrapper on top of functools.lru_cache.
4. Every time a function is decorated with @plugins_cache, a callback to the PLUGINS_CACHE_CLEAR must be added, such that the cache is cleared when that action is called.
5. Import hooks.utils.plugins_cache in hooks/__init__.py such that the function is available in plugins.
6. Make sure that the plugins_cache decorator is documented in https://docs.tutor.edly.io/reference/api/hooks/index.html
7. Migrate ENV_PATCHES_DICT to a _env_patches_dict function that is decorated with hooks.plugins_cache.