Derived from elastic/apm#138.

Add support for the next batch of central (AKA remote) config options. At the moment the next batch consists of the following options:

1. CAPTURE_BODY
2. TRANSACTION_MAX_SPANS (see Agent Configuration in Kibana, GA apm#138 (comment) regarding backend validation of this option's value)

We should also consider marking in docs which options are reloadable and via which sources (if we want to support reloadable sources other than central configuration - for example Microsoft.Extensions.Configuration)