[cygnus-ngsi] NGSLD support implementation

[anmunoz]

From UPM we have been working with FIWARE Foundation for trying to define a first development for providing NGSI-LD support to Cygnus. This pull request incorporates this new feature by adding new classes for NGSI-LD that works well with the current version of Cygnus. We have been working in the source part for allowing to build NGSI-LD objects for NGSI-LD notifications, using a Modified versión of NGSIRest Handler, and the first sink provided for this implementation is the NGSILDPostgresqlSink.

[fgalan]

Related issue #1875

[fgalan]

Thanks for your contribution!

Taking into account the feedback provided in issue #1875, we think it should be better to package the new notifications handler and sinks in a new "bundle" (not sure of the term... some parts of the documentation named this "sub-module", eg. https://github.com/telefonicaid/fiware-cygnus#install).

I mean, a new subdirectory cygnus-ngsi-ld/ with a parallel structure and way of working as the existing cygnus-ngsi/, relyging in cygnus-common/ for the common parts (have a look on how cygnus-ngsi includes cygnus-common in its pom.xml file https://github.com/telefonicaid/fiware-cygnus/blob/master/cygnus-ngsi/pom.xml#L70, cygnus-ngsi-ld should use a similar approach).

Note that all Cygnus bundles/sub-modules are supposed to have RPM generation artifacts (in spec/SPECS subdirectory). However, with this one I think that just providing docker container generation artifacts is enough (that stuff shoud be in docker/cygnus-ngsi-ld).

Finally, with regards to documentation, the place is doc/cygnus-ngsi-ld. New files should be included in a new "section" of the mkdocs.yml file.

[anmunoz]

This PR was replaced by PR #1885