Add query string to body for anon STS POST

So this PR will solely affect anonymous requests using STS. Before, the request would put the query string into the URL path. However, all requests for STS are modeled as a post and that data should be in the body. This PR removes the query string from the URL path and adds it to the body. I tested it out using some of the different ways to assume a role and it looks like it is working fine.

cc @danielgtaylor @jamesls