Added mock for opensearchpy create function

[sdhoyt]

Sorry if you're not accepting external contributions. I started using this library and noticed it doesn't currently have opensearchpy's create function that I need mocked. I figured I'd submit my changes and see.

Changes

• Added mock for opensearchpy's OpenSearch.create method
• This method creates a new document if one does not already exist or returns a 409 response if the document already exists.
• Added 4 unit tests for new method