[Feature Request] SAML authentication

## Overview
- `Summary: ` It would be great if Insomnia supported SAML 2.0 as an authentication method to use.

## Other Notes

We have some legacy (i.e. SOAP) systems that use SAML for authentication and identity. They don't support multiple forms of authentication, so testing them is not as simple as passing an admin token or HTTP Basic authentication mechanism.

It would be great if we could use Insomnia to test a request backed by SAML. I don't know the work involved in doing this, but in terms of related material to understand it, here's what I dug up:

* Auth0 SAML Configuration - https://auth0.com/docs/protocols/saml/saml-configuration
* Wikipedia - https://en.wikipedia.org/wiki/SAML_2.0
* NPMJS Enterprise SAML configs - https://npme.npmjs.com/docs/up-and-running/auth/saml.html

Start by reviewing the linked Auth0 SAML Configuration, Wikipedia, and NPMJS Enterprise SAML references to understand the requested SAML 2.0 flow. The issue is complete when Insomnia can use SAML authentication for requests to legacy SOAP systems; no implementation files or tests are identified.

Written by the indexing model from the issue text.