Add start and end parameters to /stats endpoint

[mrkhan]

• [x] use existing endpoint with optional start and end dates as query parameters (path parameters could not easily made to be optional)
• [x] parameter names: startdate, enddate, i.e. ?startdate=2017-10-01T04:00&enddate=2017-11-01 (consistent with the existing /reports endpoint )
• [x] both date parameters are optional
• [x] they should be included in the result only if they are explicitly set in the query
• [ ] startdate or enddate in the result should be in milliseconds-precision (mirroring the precision of the data) in UTC
• [x] date time format: iso-formatted
• [ ] startdate: inclusive, enddate: exclusive
• [x] document endpoint and path/query parameters via Swagger / OpenAPI

Example output for the case of an explicit enddate:

{
          "changesets": 65009011,
          "users": 3003842,
          "roads": 45964973.0494135,
          "buildings": 844294167,
          "edits": 1095091515,
          "latest": "2023-03-20T10:55:38.000Z",
          "hashtag": "*",
          "enddate": "2017-10-01T04:00:00.000Z"
}