Feature request: API/intake for setting client.geo.location

**Is your feature request related to a problem? Please describe.**

Services may know the geolocation of their clients, and that could be useful for map visualisations and aggregations. e.g. https://discuss.elastic.co/t/how-to-adjust-apm-client-setlabel-index-field-type-to-geo-point/226994 describes a use case for indexing the geolocation of IoT devices communicating with an IoT API/service.

**Describe the solution you'd like**

1. Extend the intake protocol with a new transaction/error context field: `context.client.geo.location`. The value may be one of:
1. `"context.client.geo.location": {"lat": -31.95, "lon": 115.86}`
1. `"context.client.geo.location": "qd66hrw"` ([geohash](https://en.wikipedia.org/wiki/Geohash))

2. Extend agents' APIs to enable users to record the geolocation of the client of a request. e.g. `apm.setClientGeoLocation({"lat": -31.95, "lon": 115.86})` or `apm.setClientGeoLocation("qd66hrw")`.

3. APM Server would record this new context in the ECS [`client.geo.location`](https://www.elastic.co/guide/en/ecs/current/ecs-geo.html) field. The value sent by the agent would be stored as-is; the field type is [`geo_point`](https://www.elastic.co/guide/en/elasticsearch/reference/current/geo-point.html), which supports both of the above formats and more.

**Describe alternatives you've considered**

Add the location as a [geohash](https://en.wikipedia.org/wiki/Geohash)-formatted string label, and either create a specific mapping for that label with the "geo_point" field type, or add an ingest node processor which renames the label to "client.geo.location".