MEDIUM: tcp: add a new tcp-request capture directive

MEDIUM: tcp: add a new tcp-request capture directive

This new directive captures the specified fetch expression, converts
it to text and puts it into the next capture slot. The capture slots
are shared with header captures so that it is possible to dump all
captures at once or selectively in logs and header processing.

The purpose is to permit logs to contain whatever payload is found in
a request, for example bytes at a fixed location or the SNI of forwarded
SSL traffic.