Add support for schema in collection API

[DiscoPYF]

ArangoDB 3.7 introduced JSON-schema validation for documents. The collection API has a new property schema.

https://www.arangodb.com/docs/stable/http/collection-creating.html#create-collection

There are several methods of the collection API client that we need to update to include the new property. Example: PostCollectionAsync, GetCollectionPropertiesAsync.

More information about this feature:

https://www.arangodb.com/docs/stable/data-modeling-documents-schema-validation.html

Schema Validation for Documents (What's New in 3.7)