Webhook forwarder API in Go

Very much a work in progress. Replicating in Go the webhook forwarder I built in Python as a way to better understand the language.

To-do

• Forward webhook function: headers and body
• Fetch acceptable IPs from GitHub meta api
• Proper 'is in' logic for IPs - GitHub returns CIDR ranges
• Parse IP fields as IPs for safety
• Env variables: TARGET_URL
• Env variables: WEBHOOK_TOKEN_SECRET
• Tests
• Coverage analysis for tests
• Go 'maintainability' analysis