[Feature Request] Add earliest/first to dateTimeSchema

[SpeckiJ]

Currently the special keywords: "now" and "latest" are supported for querying via time:

ogcapi-connected-systems/api/part1/openapi/parameters/datetimeSchema.yaml

Line 17 in 2904f1c

enum: [now, latest]

For clients it would be nice to also have earliest/first as a reference point. Without this there is no real straightforward way to get the earliest observation in a datastream. Especially for giving an overview of the data this is often required.

While the Datastream itself also has a phenomenonTime, this is often only a very broad timeframe (e.g. the length of the overall measurement campaign) and not necessarily the actual timeframe of the first observation.

[alexrobin]

Discussed during 09/19 telecon.
Ok to add earliest. (mandatory like other options in the enum)

[autermann]

Discussed 01/23: remove the now keyword and just keep latest.

Finding out the earliest values is trivial, as the DataStream has temporal extents for result and phenomenon times.