Using pluggy for managing Plugins

## Enhancement proposal
Pluggy: https://pluggy.readthedocs.io/

Currently how out plugin system works is
- Get the list of plugins from the user config file
- Add them to the global namespace, so when the user does `from manim import *` the plugin will also be imported.
- Has no possible way to extend parts of the library without monkey patching or making a new subclass out of the original one. See how `manim-onlinetex` does this currently, https://github.com/ManimCommunity/manim-onlinetex/blob/38ed76c55f387e9205beeb28307a634b46691df1/src/manim_onlinetex/manim_onlinetex.py#L109

With pluggy in place, we can decide the parts of the library which can be extendable without subclassing, see an example. https://pluggy.readthedocs.io/en/latest/#a-toy-example
And it feels like we shouldn't reinvent the wheel rather than using something which is already available.

## Additional comments
This would kinda require all the plugins to update to use pluggy. Let the previous implementation of importing to global namespace stay, for backward compatibility.

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Proof of concept
- #2635

Review the existing plugin behavior described in the issue, the manim-onlinetex example, Pluggy's toy example, and proof of concept #2635. Define the supported extension points and the backward-compatibility and plugin-migration criteria before implementation; the issue does not name specific repository files or tests.

Written by the indexing model from the issue text.