Implements /documents API

[sinedied]

• Implement /documents/<id> GET API
• API connects to blob storage to retrieve a given document
  • Use Functions v4 blob storage connector
• (if not implemented) Add blob storage upload to POST /upload endpoint
• Complete an api.http file to test this API using REST Client extension

Note: this is for later, as an enhancement to the existing app, and not mandatory for initial release.

[sinedied]

Blob connector cannot be used, blocked by: Azure/azure-functions-nodejs-library#72