feat(c/v3-engine): introduce `jwkFromUrlFromEnv`

### Component

c/v3-engine

### Is your proposal related to a problem?

`jwkFromUrl` is hard-coded.

```yaml
kind: AuthConfig
definition:
mode:
jwt:
key:
jwkFromUrl: "https://hard-coded-url.com"
```

### Describe the solution you'd like

jwk URL can be different from prod, stg, dev, etc.

```yaml
kind: AuthConfig
definition:
mode:
jwt:
key:
jwkFromUrlFromEnv: JWK_URL
```

The proposal targets c/v3-engine's AuthConfig JWT key configuration; start by tracing the existing `jwkFromUrl` entry point and its environment/config handling. Done means the YAML form can source the JWK URL from `JWK_URL` while preserving the existing URL form, with coverage for both forms.

Written by the indexing model from the issue text.