Provide ability to extract Geo information from RUM agent's http request

In some use-cases it's desirable to extract geolocation information from http headers instead of using GeoIP. For Example to protect users' privacy by replacing the client ip header with an arbitrary string.

To provide this ability the APM server can look for certain headers on the request to extract the geolocation information. E.g. `X-Geo-Country` header could provide the country. We should investigate which headers are relevant/useful in this context to provide the right balance.

Please see this [thread](https://discuss.elastic.co/t/rum-enrich-document-with-http-request-headers/266844) for more detail.