ci: add proxy usage test cases

Add two test jobs:

- test-token-with-proxy-succeeds: gcp-auth-provider should honor
  http_proxy or https_proxy variables and use it if pipeline runs on a
  runner with no direct Internet access.

- test-token-with-proxy-unavail-fails: gcp-auth-provider should fail if
  a proxy is set but the proxy is not available.

The first test currently passes as the Gitlab runner has direct Internet
access, when proxy environment variables are honnored the test would
fail if proxy handling were defective.

The second test currently fails as http proxy variables are not honored
and gcp-auth-provider still uses direct Internet access and returns a
status 200 instead of 500 as expected.