Add --cog-path CLI argument

[cswimr]

Description of the changes

This PR adds an argument to redbot that takes a list of strings (can be used multiple times for more than one value), validates that the path provided exists, and adds the path via self._cog_mgr.add_path(), so that cogs can be loaded from that path.

Closes #6506

Have the changes in this PR been tested?

Not thoroughly

[Flame442]

This PR does not behave as outlined in the issue. self._cog_mgr.add_path() is the method used by [p]addpath to add a new permanent cog path to the bot. The issue described this flag as a temporary path which would only exist as long as the path was present. The method does not duplicate paths, so it is not a huge problem, but if we want this flag to specifically be for temporary paths then a new method for doing so will be needed.

[cswimr]

Oops, good catch! I'll take another look at this.

[cswimr]

So, I propose the following:
Temporary paths would be stored in config, separately from ones added via [p]addpath. This is the safest place that I can think of to store them. When the bot is started, that config value would be overwritten with the contents (if any) of the --cog-path CLI argument. If nothing is passed to --cog-path, then the config value is emptied. A new function would have to be added to CogManager to retrieve these temporary paths, and CogManagerUI would have to be updated to check this in [p]paths. Is there a better way Red has to do something like this? It feels kind of clunky imo but it'd work I think.

[Flame442]

Why not just store it in memory during the run the flag is used? Why does the disk need to be used at all?