Introduce DCDocumentStore and allow it to read documents

[tstadel]

In order to communicate with DocumentStores in DC we need a readonly DocumentStore implementation that is compatible with all retrievers incl. ElasticsearchRetriever.

First we want it to be initialized and able to retrieve all documents without actually querying.
It should be initialized like that:
doc_store = DCDocumentStore(api_key="apikey", index="my_index", workspace="default")

The following methods need to be implemented:

method	API Call
init	GET api/v1/workspaces/{workspace_name}/indexes/{index_name}
get_all_documents	GET api/v1/workspaces/{workspace_name}/indexes/{index_name}/documents
get_all_documents_generator	POST api/v1/workspaces/{workspace_name}/indexes/{index_name}/documents-stream
get_document_by_id	GET api/v1/workspaces/{workspace_name}/indexes/{index_name}/documents/{document_id}
get_documents_by_id	GET api/v1/workspaces/{workspace_name}/indexes/{index_name}/documents/{document_id}
get_document_count	POST api/v1/workspaces/{workspace_name}/indexes/{index_name}/documents-count

Documents are supposed to be fetched via paging.