Cosmos DB: Add _filter support in GraphQL resolver

[Aniruddh25]

See this RFP: https://github.com/Azure/project-hawaii/issues/11

[Aniruddh25]

This issue is to support filtering using a string with the default parameter '_filter'.

The string value is of the same format as the $filter clause in SQL REST API. The PRs to parse that and support filtering have been checked in. Let me know if you have any questions.