Add an e2e style test to verify meaningful RPC error when using insecure transport creds with A97 JWT call creds #8635
Description
As part of gRFC A97, we are adding support for JWT call credentials that can be configured via xDS. We are also adding a general purpose mechanism where we can support multiple call credentials specified in the bootstrap configuration.
If one of these call credentials requires a secure transport, but an insecure transport is configured, we expect the RPC to fail.
We need an e2e style test to verify that when this happens, a meaningful error is returned to the user as part of the RPC.