Add support for external-callout

[diksha-sf]

Validated using pre-release version: v6.1.0.dev3

[atulikumwenayo]

1. I think we should expect any string format for the input, and any string format for the server response, we shouldn't assume what they are: json, text, xml, etc. This also means we shouldn't have a default value for Content-Type header
2. Exposing only one method (request) and removing callout_json would suffice.
3. It should be status_code not http_status_code
4. Should we let the user control the response timeout, maybe with a hard max that we don't allow them to exceed?

[diksha-sf]

@atulikumwenayo addressed the PR review comments 1-3. For 4, I synced with @sarasavilli-sf, this would need more work. Created WI - W-23683713