Add support for single port "ranges" in flow descriptions

[pudelkoM]

This PR adds support for single port "ranges" like 8888-8888 in flow descriptions. It covers only the simple case with a single port, not actual ranges. It's a first step towards full support and fixes the errors we're encountering right now:

Error log:

time="2021-12-02T07:06:21Z" level=info msg="Flow Description is: permit out udp from 192.168.102.122/32 to assigned 6128-6128" func="main.(*pdr).parsePDI" file="/pfcpiface/parse-pdr.go:231"
time="2021-12-02T07:06:21Z" level=info msg=from func="main.(*ipFilterRule).parseFlowDesc" file="/pfcpiface/parse-sdf.go:109"
time="2021-12-02T07:06:21Z" level=info msg="[permit out udp from 192.168.102.122/32 to assigned 6128-6128]" func="main.(*ipFilterRule).parseFlowDesc.func1" file="/pfcpiface/parse-sdf.go:92"
time="2021-12-02T07:06:21Z" level=info msg="[permit out udp from 192.168.102.122/32 to assigned 6128-6128]" func="main.(*ipFilterRule).parseFlowDesc.func1" file="/pfcpiface/parse-sdf.go:105"
time="2021-12-02T07:06:21Z" level=info msg="[192.168.102.122 32]" func="main.(*endpoint).parseNet" file="/pfcpiface/parse-sdf.go:28"
time="2021-12-02T07:06:21Z" level=info msg=to func="main.(*ipFilterRule).parseFlowDesc" file="/pfcpiface/parse-sdf.go:109"
time="2021-12-02T07:06:21Z" level=info msg="[permit out udp from 192.168.102.122/32 to assigned 6128-6128]" func="main.(*ipFilterRule).parseFlowDesc.func1" file="/pfcpiface/parse-sdf.go:92"
time="2021-12-02T07:06:21Z" level=info msg="[permit out udp from 192.168.102.122/32 to 0.0.0.0/0 6128-6128]" func="main.(*ipFilterRule).parseFlowDesc.func1" file="/pfcpiface/parse-sdf.go:105"
time="2021-12-02T07:06:21Z" level=info msg="[0.0.0.0 0]" func="main.(*endpoint).parseNet" file="/pfcpiface/parse-sdf.go:28"
time="2021-12-02T07:06:21Z" level=info msg="dst port parse failed  strconv.ParseUint: parsing \"6128-6128\": invalid syntax" func="main.(*ipFilterRule).parseFlowDesc" file="/pfcpiface/parse-sdf.go:143"