Import data models from old importer-script repository

[danduk82]

This PR adds

• data models for the geoserver types (workspace, datastore, featuretype, etc)
• a dedicated http client service with dedicated url endpoints for each resource type

Breaking changes
several input parameters of the GeoServerCloud class methods have been renamed:

• workspace -> workspace_name
• datastore -> datastore_name

The reason for the breaking change, is that now that we have the types, a workspace is an instance of the geoservercloud.models.Workspace class, and workspace_name is a string that represent the name of the workspace. This applies to all the types.