PR: Migrate completion plugin to the new API

[andfoy]

Description of Changes

This PR migrates the completion plugin (former CompletionManager) to the new API. As part of this PR, we also introduce a CompletionProvider interface, from which all completion providers inherit. The new CompletionPlugin will take all the providers (whom are registered via entrypoints), initialize them and manage them.

• Wrote at least one-line docstrings (for any new functions)
• Added unit test(s) covering the changes (if testable)

• Included a screenshot or animation (if affecting the UI, see Licecap)

Issue(s) Resolved

Fixes #12192

Affirmation

By submitting this Pull Request or typing my (user)name below,
I affirm the Developer Certificate of Origin
with respect to all commits and content included in this PR,
and understand I am releasing the same under Spyder's MIT (Expat) license.

I certify the above statement is true and correct: @andfoy

[ccordoba12]

Thanks a lot @andfoy for your hard work on this one! I left an initial review for you.

[ccordoba12]

@andfoy, this is a new review for you.

[dalthviz]

Regarding the Kite support I did an initial test but seems like enabling disabling kite is not being properly updated in the status bar:

After unchecking the enable kite config (and after restarting Spyder) I still see the Kite logo and base text in the status bar:

Enabling/disabling Kite:

[ccordoba12]

Hey @dalthviz, thanks for your feedback. I'm going to merge this PR now so we can use the observer pattern one (#14852) to finish the migration of the pending plugins. So please open a new issue about the problems you find with the Kite provider and @andfoy will take care of it in another PR.

[ccordoba12]

Thanks a lot @andfoy for this!! It's a fantastic improvement!

[steff456]

In my testing of this PR all the options of the preference pane are working as expected! Thanks @andfoy for this work