feat: Allow custom origin header for Websocket

[hamann]

Summary

The issue described in #641 (comment) was caused by an hardcoded origin header at

gatus/client/client.go

Line 265 in 1e431c7

Origin = "http://localhost/"

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

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

[TwiN]

While I agree that adding support for passing the origin header is worth it, I think perhaps we shouldn't add it by default here but instead add a note in the docs mentioning that users may have to specify an Origin header.