Create a specific configuration for classification only (#2689)

In some scenarios, you might not be interested in flow metadata or
flow-risks at all, but you might want only flow (sub-)classification.
Examples: you only want to forward the traffic according to the
classification or you are only interested in some protocol statistics.

Create a new configuration file (for `ndpiReader`, but you can trivially
adapt it for the library itself) allowing exactly that. You can use it
via: `ndpiReader --conf=example/only_classification.conf ...`

Note that this way, the nDPI overhead is lower because it might need
less packets per flow:
* TLS: nDPI processes only the CH (in most cases) and not also the SH
  and certificates
* DNS: only the request is processed (instead of both request and
  response)

We might extend the same "shortcut-logic" (stop processing the flow
immediately when there is a final sub-classification) for others
protocols.

Add the configuration options to enable/disable the extraction of some
TLS metadata.

tests/cfgs/classification_only/config.txt

@@ -0,0 +1 @@
+--conf=../../../example/only_classification.conf