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

Dockerfile

@@ -6,6 +6,6 @@ WORKDIR /app
 COPY . /app
 
 RUN go mod download
-RUN go build ./cmd/pcap-broker
+RUN go build .
 
 ENTRYPOINT ["./pcap-broker"]