Add webhook package

[kamaln7]

This adds a webhook package to be used by DigitalOcean users directly and consulted as a reference implementation of the request signing spec.

Example usage:

http.HandleFunc("/webhook", func(w http.ResponseWriter, req *http.Request) {
   // only allow POST requests
   if req.Method != http.MethodPost {
       w.WriteHeader(http.StatusMethodNotAllowed)
       return
   }


   // verify request authenticity
   err := webhook.VerifyHTTPRequest(req, "webhook-secret", webhook.VerificationOpts{})
   if err != nil {
       w.WriteHeader(http.StatusForbidden)
       w.Write([]byte(err.Error()))
       return
   }


   // read the request body
   eventJSON, err := io.ReadAll(req.Body)
   if err != nil {
       w.WriteHeader(http.StatusInternalServerError)
       w.Write([]byte(err.Error()))
       return
   }
   defer req.Body.Close()
  
   eventName := req.Header.Get(webhook.HTTPHeaderEventName)
   // parse the request body json into a structured type using the eventName to determine
   // which type of event it is
   // ...
})

[andrewsomething]

For posterity, I wanted to carry over my comment from a discussion of this elsewhere. It would be great to have some package level comments that discuss the signature format and perhaps include the example from the PR description.

[gitguardian]

⚠️ GitGuardian has uncovered 4 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

Since your pull request originates from a forked repository, GitGuardian is not able to associate the secrets uncovered with secret incidents on your GitGuardian dashboard.
Skipping this check run and merging your pull request will create secret incidents on your GitGuardian dashboard.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	GitGuardian status	Secret	Commit	Filename
9440723	Triggered	PostgreSQL Credentials	aca056d	databases_test.go	View secret
9440723	Triggered	PostgreSQL Credentials	aca056d	databases_test.go	View secret
9440724	Triggered	PostgreSQL Credentials	aca056d	databases_test.go	View secret
9440724	Triggered	PostgreSQL Credentials	aca056d	databases_test.go	View secret

🛠 Guidelines to remediate hardcoded secrets

1. Understand the implications of revoking this secret by investigating where it is used in your code.
2. Replace and store your secret safely. Learn here the best practices.
3. Revoke and rotate this secret.
4. If possible, rewrite git history. Rewriting git history is not a trivial act. You might completely break other contributing developers' workflow and you risk accidentally deleting legitimate data.

To avoid such incidents in the future consider

• following these best practices for managing and storing secrets including API keys and other credentials
• install secret detection on pre-commit to catch secret before it leaves your machine and ease remediation.

---

^{🦉 GitGuardian detects secrets in your source code to help developers and security teams secure the modern development process. You are seeing this because you or someone else with access to this repository has authorized GitGuardian to scan your pull request.}