Returns a batch of EVSE dynamic statuses (availability + ad-hoc price) that changed since
the supplied since cursor, in DATEX II format. Omit since to fetch a full snapshot.

Paging is cursor-based: pass the returned publicationTime back as since on the next request and repeat
until an empty batch is returned. Because rows are de-duplicated after paging, a batch may contain fewer
unique EVSEs than perPage; a short (non-empty) batch does NOT mean end-of-data — only an empty batch does.
When a batch is empty the request's since is echoed back as publicationTime (the cursor does not advance).

The since/publicationTime value is an opaque, full-precision datetime cursor
(YYYY-MM-DD HH:MM:SS.fffffffff); pass it through verbatim without reformatting or truncating.