Add endpoint support for individual read receipts

[brettp]

Added POST /v1/receipt/{number} endpoint to support read receipts for individual messages, as opposed to all received as currently exists with /receive. Complements functionality raised in #431 and #524.

Accepts a payload like:

{
  "receipt_type": "read",
  "recipient": "+12345678900",
  "timestamp": 1715386986897
}

[bbernhard]

Many thanks!