feature(webhook): add template and content type

[josedonizetti]

1. Explain what the PR does

Fix #3448

This PR adds support to gotemplate and contentType to the webhook printer.

2. Explain how to test it

sudo ./dist/tracee -p examples/policies/dig.yaml -o webhook:"http://localhost:8081?gotemplate=/path/to/template/test.tmpl&contentType=application/json"

example tmpl:

{
        "test": {{ .EventName }}
}

3. Other comments

[geyslan]

to @geyslan from the future: This affects #3612.

[geyslan]

LGTM