[External Tools] Support environment variables for LTI 1.3

[agschmid]

For LTI 1.1 there was support for environment variables (https://canvas.instructure.com/doc/api/file.tools_xml.html#test_env_settings) so when beta/test refreshed, our LTI tools would point to beta/test on the beta/test servers instead of production. We've used the environment variables for years.

We're in the process of switching to LTI 1.3 and were told that there is no support for environment variables, according to Instructure "it doesn't seem like we've seen much need for this from our end users so far until now."

I know we are not the only ones using the environment variables and hope that Instructure will decide to support environment variables for LTI 1.3.