Metadata plugins - #1700

Addresses issue #1899

Adds a new and hopefully simpler method of registering plugins by specifying entry points in the plugins pyproject.toml. This means that you can just pip-install the plugin and PYME will find it.

@csoeller @barentine - this should nicely handle the plugin installation workflow for pip based plugins discussed in email.

@zacsimile - Have already ported ch-shrinkwrap
@barentine - Haven't done anything to nep-fitting

Docs below:

Declare entry points in your package's pyproject.toml. PYME discovers them automatically on startup with no
file copying. This works identically for pip, conda, and editable installs.

    [project.entry-points."pyme.plugins.recipes"]
    myplugin        = "mypackage.recipe_modules"
    myplugin_extra  = "mypackage.extra_modules"

    [project.entry-points."pyme.plugins.visgui"]
    myplugin = "mypackage.visgui_modules"

    [project.entry-points."pyme.plugins.dsviewer"]
    myplugin = "mypackage.dsviewer_modules"

    [project.entry-points."pyme.plugins.fit_factories"]
    myplugin = "mypackage.fit_factories"

The entry point name (left-hand side) is arbitrary and ignored by PYME; the value is the fully qualified module
path. An optional :attr suffix (e.g. mypackage.module:some_func) is accepted but only the module path is used.