Support DynamoDB.DeleteItem in DynamoDB.mock()

[mark-reese]

The DynamoDB mocking already supports CreateTable, PutItem and GetItem. It would be useful to have support for DeleteItem as well.

The ASK SDK DynamoDB Persistence Adapter supports delete. If a skill uses this adapter, virtual-alexa cannot be used to test this delete functionality without a workaround.

https://github.com/alexa/alexa-skills-kit-sdk-for-nodejs/blob/2.0.x/ask-sdk-dynamodb-persistence-adapter/lib/attributes/persistence/DynamoDbPersistenceAdapter.ts

One possible workaround is to use nock inside the tests to mock the call to DeleteItem (which is what the bespoken DynamoDB. class does internally for the methods it does implement).

It would be much nicer if DeleteItem was supported within DynamoDB.class

If you approve this feature, I can submit a PR for this work.

[allthepies]

Hi, I added a deleteItem operation to the mock Dynamo as I needed it for some of my skills.
Hopefully bespoken will approve the pull request -> #139