hatchling metadata hook support

[alexrashed]

Motivation

#35 adds support for Hatchling as a build backend for the automatic discovery of plugins and to populate the plux.ini.
However, in contrast to setuptools, hatchling does not natively support fetching the entry-points from an external file. Instead hatchling provides an explicit "metadata hook" to allow simply dynamically modifying the project metadata.
This PR extends #35 by adding an explicit PluxMetadataHook which takes care of populating the entrypoints of the project with the plux.ini file.

Changes

• Extends the hatchling integration in plux.build.hatchling with a simple PluxMetadataHook.

TODO

• Modify the README.md

[dfangl]

LGTM! Makes sense, and the additional tests are great!