ElasticSearch backend support in pycsw (Q7)

[kalxas]

Description

The plan is to work on ElasticSearch backend for Resource Catalogue.
Work to be continued from previous Q (backend abstraction)

Acceptance criteria

• Resource Catalogue uses ElasticSearch as backend.

Contributes to

• Support ElasticSearch as a Resource Catalogue backend in pycsw #26
• https://github.com/EOEPCA/roadmap/issues/315

[kalxas]

geopython/pycsw#1158