Improve extensions information

[sylvlecl]

• Do you want to request a feature or report a bug?

Feature.

• What is the current behavior?

The user can query the list of extensions, but he does not get any information about it.

>>> pn.get_extensions_names()
['activePowerControl',
 ...,
 'xnode']

• What is the expected behavior?

We could provide additional information about the extensions, for example:

>>> pn.get_extension_description('activePowerControl')
{
   description: 'Provides information about the participation of generators to balancing',
   attributes: ['participate', 'droop']
}

The return type could be a small dedicated class.

Alternatively, it could instead return a dataframe for all extensions.