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.

[kadaan]

@TwiN It seems as the code as in this PR ensures that the current behavior, which users might rely on, continues to functions as it has. I'm unsure if ceasing the specify the Origin header will end up being a backwards incompatible change. What is the benefit of no longer specifying the Origin header on any websocket call unless it is explicitly specified?

[TwiN]

@kadaan I don't personally have any websocket endpoint handy to test this on; I'm just trying to avoid backward incompatible changes to the best of my ability. Could you investigate this? Once the conflicts are resolved, I wouldn't mind merging this.