Research blocking all POST requests

[qqss88]

• *We're going to tackle the work in Implement download endpoint limits #3651 first and once that work is complete determine whether we need to complete this ticket as well.

POST api will catch more attentions from hacker. We'll research the possibility of changing the request mode from POST to GET and the influence on our system.

the code link is here:
https://github.com/fecgov/openFEC/blob/develop/webservices/resources/download.py

• Test get method for downloads
• Throw 405 error for all Post requests https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/405 (check logs for examples of POST /. Include Allow` header. Research best practices for using the Allow header to indicate allowed HTTP methods. See https://evertpot.com/http/405-method-not-allowed