Make pcap-broker go installable (#8)

This mainly adds a main.go file in the project root so it's easier to
install pcap-broker using `go install`:

    $ go install github.com/fox-it/pcap-broker@latest

Or install the main branch:

    $ go install github.com/fox-it/pcap-broker@main

main.go

@@ -0,0 +1,9 @@
+package main
+
+import (
+	pcap_broker "github.com/fox-it/pcap-broker/cmd/pcap-broker"
+)
+
+func main() {
+	pcap_broker.Main()
+}