Implement hook system

[danielfdsilva]

Check the docs at: https://github.com/developmentseed/stac-manager/blob/feature/plugin-hooks/docs/PLUGINS.md#hooks

[danielfdsilva]

It includes the docs from the #28 branch

[j08lue]

This is a nice feature for folks who want to add a custom plugin and make it easy for their users to reference it in the app.

The "real world example" and helper function for this case are excellent.

Excited to see how this gets picked up.