iperf3: add shared libiperf library and link iperf3 dynamically

Add library for creating own functions with iperf3 functionality.
Example: https://github.com/esnet/iperf/blob/master/examples/mis.c
This library is needed by python3-iperf3.

Build iperf3 binary with dynamically linked libiperf3. However, still
build iperf3-ssl as static binary due to a lack of shipping two libiperf
versions.

Signed-off-by: Nick Hainke <vincent@systemli.org>