Function try_parse_sts_request 

pub fn try_parse_sts_request(
    query: Option<&str>,
) -> Option<Result<StsRequest, ProxyError>>

Try to parse an STS request from the query string.

Returns None if the query does not contain Action=AssumeRoleWithWebIdentity (i.e., this is not an STS request). Returns Some(Ok(..)) on success or Some(Err(..)) if it is an STS request but required parameters are missing.