Sidetree document metadata

[troyronda]

• enable metadata related to a Sidetree document
• metadata must be separated from the document.
  • Sidetree REST endpoints must be able to apply different security policy.
  • DCAS and Blocks REST APIs must be able to apply different security policy, such that metadata related information is not returned unless security level is met.
• the metadata probably should be stored in a different DCAS collection. usage of a different collection also helps with the security policy point.