ESchemas now accepts the entity types as generic type parameter.
SchemaEntityTypes is now the subset of "entities" property's keys which
are strings. This is necessary because when the keyword "keyof" is used
on a generic type it returns an union of strings, numbers and symbols.
We only want to keep strings.

Demonstrates how to create relationships to existing entities

The rqlClient is used so often that we can expect it to be available
when creating the data provider.