Add schema to crawl route

[ashley-evans]

Description

Currently no validation is performed on the message's that get sent to the /crawl route. Therefore, any message with any body structure can be sent to that route and place on the SQS queue.

The crawl-urls lambda function does validate that the input records are valid against a schema, however, it would be useful to reject these requests at the point of the API Gateway such that a correct response code (400) is returned.

Acceptance Criteria

AC01

• A schema matching the one applied within the crawl-urls lambda should be applied to any request made to the Buzzword API Gateway /crawl route
• An invalid request should result in a 400 response code
• Valid requests should be added to the SQS queue (Current behaviour)

AC02

• All the changes required to achieve the above AC should be completed using SAM template changes
• i.e. no manual changes required in the AWS Console