feat: Allow custom origin header for Websocket

[hamann]

Summary

The issue described in https://github.com/TwiN/gatus/issues/641#issuecomment-1980446434 was caused by an hardcoded origin header at https://github.com/TwiN/gatus/blob/1e431c797ad265b43269d1dfe270f1acd37d24fa/client/client.go#L265 which got validated by the websocket server and returend 403 (bad status, from https://github.com/golang/net/blob/ab271c317248ea0f18481852f96d12d5eca05cf8/websocket/hybi.go#L440-L445) This PR allows to customize the origin header. Example

endpoints:
  - name: websocket
    url: "wss://example.org"
    interval: 10s
    headers:
      Origin: "http://gatus.example.org"
    conditions:
      - "[CONNECTED] == true"

Fixes #641

Checklist

• [X] Tested and/or added tests to validate that the changes work as intended, if applicable.
• [ ] Updated documentation in README.md, if applicable.