Prevent CORS from blocking DELETE in mock-api server

[jaretcantu]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

The DELETE (and I assume PATCH and PUT) request methods are being blocked by CORS in the mock server:

Access to XMLHttpRequest at 'http://localhost:8080/mock/channel_points/custom_rewards?broadcaster_id=XXXX&id=XXXX'
from origin 'http://localhost' has been blocked by CORS policy:
Method DELETE is not allowed by Access-Control-Allow-Methods in preflight response.

Description of Changes:

• Set the additional HTTP methods in a header in the mock-api server
• This is the same Access-Control-Allow-Methods list from the websocket server (internal/events/websocket/mock_server/manager.go)

Checklist

• My code follows the Contribution Guide
• I have self-reviewed the changes being requested
• I have made comments on pieces of code that may be difficult to understand for other editors
• I have updated the documentation (if applicable)