Added utc_start_dt and utc_end_dt as start and date, respectively, to the API response

• Added utc_start_dt and utc_end_dt as start and date, respectively, to the API response where mediawiki is the authority /api/v1/experiments?format=config&authority=mediawiki) and also for instruments /api/v1/instruments)
• start and end dates for both instruments and experiments in the API response are in RFC 3999 UTC form
• Taken the opportunity to reduce the payload of the /api/v1/instruments response by removing the non-technical/unneeded fields
  • Fields included in the API response: slug, start, end, sample_unit, sample_rate, stream_name, schema_title, contextual_attributes

• Tests cases have been updated accordingly

This change is fully related to https://gerrit.wikimedia.org/r/c/mediawiki/extensions/MetricsPlatform/+/1177961 with which MediaWiki will start considering start and end dates to filter inactive instruments/experiments based on those dates. This change must be deployed first.

Future active instruments or experiments are not included in the response yet (as we were doing before !220 (merged)). We'll do that in a follow up ticket once MediaWiki change is merged and starts checking start and end dates

Bug: T399292