Setup pluggy with locking plugin

[soapy1]

Fixes #929

Description

This set's up conda-lock to be run as a plugin. By running the locking functionality as a plugin, opposed to a builtin action, we will be able to more easily swap out locking implementations. This is helpful for developer experimentation and providing users with another way of customize their conda-store install.

Some things that this PR adds:

• adds pluggy as a dependency
• adds a plugin manager for collecting and loading plugins
• moves locking action to the conda-store lock plugin
• adds a plugin context object for sharing required state and functionality between plugin runs
• adds a config option to set the default lock plugin

For a more complete demo of how this approach can be extended, see main...soapy1:conda-store:plugin-demo-pluggy. For a more complete description of what is added by this demo see the conversation in GH.

Next steps

fast follow pr's

• persist the default lock plugin selection to the settings db table (and pull the default value from there)

opportunities to extend the use of plugins

• allow users to define their own plugins
  • (once the plugin use case is more flushed out) document how to build user plugins
• it would be nice to move some other parts of conda-store to be plugins. For example, the storage backends, or other actions
• try out other lock backends

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

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[jaimergp]

Hm, I was expecting the implementations to live in a different abstraction level, not so close to the actual code. For reference, this is how conda does it.

[jaimergp]

Thanks @soapy1! I like what I'm seeing, but I'm wondering if we could leverage some "community knowledge" and steal what conda is doing for their hooks. They have one more layer in between the plugin and the hook, and the best part if that they have documented it veeery well. Do you think you can take a look at their setup and see what things we could benefit from? 🙏

[jaimergp]

This LGTM assuming all tests are passing. Thanks for the hard work @soapy1!