Update PDTF API specification

[amolpednekar]

Updates to PDTF API specification

• Updated format of open-api-spec from JSON to YAML (superset of JSON, more readable, allows adding comments)

• Fixed errors in schema to allow rendering in a swagger editor

  • Updated $ref section to point to the Github pdtf transactions schema instead of non-existent hsbg links
  • Added OAPI tag "transactions"

• Replaced API section: transactions (previously properties)

  • This is inline with how [RICS]((https://github.com/RICS-Data-Standard/RDS/blob/main/JSON/RICS%20Data%20Standard%20Version%203.3.3.pdf) have defined their schema *Allows to get property pack in the context of a transaction by sending transaction ID as a query parameter (allowing uniqueness)
  • Still allows to query by UPRN but should return multiple transactions

• Merged getState and getStateAsHtml into a single API with configurable contentType

Action items for @edmolyneux

• [ ] Add schema definition: /transactions/{id} API (latest state API)
• [ ] Add schema definition: /transactions?uprn=xyz&status=active API

[amolpednekar]

@edmolyneux - I have updated the spec based on review comments & meeting discussions on Thursday (24 March 2022). Please review.

• Requesting you to add two schema definitions
  • /transactions/{id} API (latest state API)
  • /transactions?uprn=xyz&status=active API

I have set them to just a string type for this PR.