feature: two-step plugin installing and removing

Summary

• feature: two-step plugin installing and removing.

Now for installation and enable plugin, there are two steps instead of one (pico.load_plugin previously).

1. Install plugin (pico.install_plugin): validate plugin, create system tables.
2. Enable plugin (pico.enable_plugin): loads plugin into instances memory, call on_start callback.

Same for a plugin removing process (pico.remove_plugin previously):

1. Disable plugin (pico.disable_plugin): call on_stop callbacks, drop plugin from instances.
2. Remove plugin (pico.remove_plugin): remove plugin meta from system tables.

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• Changes should be cherry-picked to 24.2: no
• Docs follow-up: docs#236 (closed), docs#237, docs#238, docs#239

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Please see the updated plugin life-cycle diagrams:

https://docs.google.com/document/d/1QMFXWhrSJ9VLFNX-jayHCzG8jk01VwtOuTOzooBBsOA/edit#heading=h.i0z7fgvh5e96