Introduce configuration to disable use of HTTP headers for identifying `client.ip`

We make use of various HTTP headers, such as X-Forwarded-For, for identifying the client IP of RUM traffic. This assumes that clients are either trusted, or APM Server is behind a reverse proxy which removes these headers from untrusted clients to prevent spoofing of `client.ip`.

We should provide configuration to disable the use of HTTP headers, for users who directly expose APM Server to untrusted users without any proxy in between.