move external python apis and server to extras_require

[sbrunato]

Get external python apis out of install_requires (usgs, ecmwf-api-client), in order to keep eodag core as light as possible. Also move server mode the same way.

Raise a well formatted error message if the external api is needed:

Access to ECMWF data requires the installation of an additional library.
Please install it via pip install ecmwf-api-client`.

And add the "complete" extras_require, giving the ability to install these external python apis with pip install eodag[complete] (also available in eodag[dev])

[avalentino]

This could be indeed a good idea, especially considering that the status of the maintenance upstream package usgs is not totally clear at the moment. It could be unmaintained (see kapadia/usgs#38).