Add an example for env_var-based credential injection (multi-service gateway)

[shaneutt]

credential_injection can source a credential from an environment variable
(env_var, mutually exclusive with value),
but no example demonstrates it. The only example,
examples/configs/security/credential-injection.yaml,
uses static value: secrets, and nothing under examples/configs uses
env_var:. So the production-shaped usage (read secrets from the environment
rather than commit them) is undocumented.

It would also help to show the multi-service gateway pattern: one listener
fronting several credentialed upstreams, each with its own env_var credential,
host routing, and path rewrites. Draft PR #1005 sketched exactly this (six APIs)
but as a personal, machine-specific config outside the examples convention; it
could be adapted.

Proposal: add examples/configs/security/credential-injection-env-vars.yaml (or
similar) that:

• injects per-cluster credentials from env_var
• fronts a couple of upstreams to show the gateway pattern
• carries the standard example header comment (so the README generator picks it
  up) plus a functional integration test under
  tests/integration/tests/suite/examples/, per the repo's example conventions

Praxis version: main (v0.5.3 line)