Created CollectionSchema and associated classes

[tjoubert]

This solves #312 CollectionSchema is a class structured according to the schema attribute as described on these pages: https://www.arangodb.com/docs/stable/http/collection-creating.html#create-collection https://www.arangodb.com/docs/3.9/data-modeling-documents-schema-validation.html#error-message

I decided to set the type of the Rule property to object in order to allow developers the freedom to pass any JSON-serializable object or plain JSON string in this attribute. The ArangoDB server will take care of validating the rule at the time that it is sent to the server.