Allow additional, configurable, properties to be dropped when SP-Anonymous is set

[paulboocock]

There are use cases where dropping properties when SP-Anonymous is set would be beneficial, such as removing the User Agent string as discussed in #111

This could be implemented at the collector level via configuration, or as a value in the SP-Anonymous header or perhaps as an enrichment (although this means the CollectorPayload would contain the value for anonymisation at Enrichment).