[Agent] Add geo metadata to agents on enrollement

As discussed with @ph @drewpost @mostlyjason and @blakerouse it'd be useful to have the [same fields present in the `add_observer/geo_metadata` processors](https://www.elastic.co/guide/en/beats/filebeat/master/add-observer-metadata.html) available from the agent. This could be exposed via an API to drive Uptime's UI, showing which geographic regions can run uptime monitoring checks.

Furthermore, it'd be great to automatically fill this data based on cloud metadata where possible, providing sane defaults for common cloud datacenters like us-east-1a on AWS etc.

Start by reviewing the linked add_observer/geo_metadata processor documentation and the agent enrollment and API entry points. Define how the geo fields should be exposed for Uptime's UI and how cloud metadata such as AWS us-east-1a should populate defaults. Done means the agent provides the agreed metadata through the API with sensible cloud-derived values.

Written by the indexing model from the issue text.