Added new feature: user-provided agency metadata to connect to any compliant data provider

[Leo]

Hi,

Request objects now have a 'load_agency_profile() method allowing the
user to Register custom agency metadata in JSON Format. This opens up
a playground for users to make requests to arbitrary data Providers
not hard-coded in pandaSDMX. Banca d'Italia might be a good
starting-point. Another might be the UK Office for National
Statistics. To see how metadata records for an agency can be
structured, see the built-in metadata in 'pandasdmx.agencies.json'.

Note that this Feature has not been released yet. Get it from the github repo.

I am happy to add metadata on new data Providers in future Releases.

Leo