[Feature Request]: Add Creduent zero-trust agent identity verification plugin for ADK

** Please make sure you read the contribution guide and file the issues in the right place. **
[Contribution guide.](https://google.github.io/adk-docs/contributing-guide/)

## 🔴 Required Information

### Is your feature request related to a specific problem?
When ADK workflows delegate tasks to sub-agents, agents lack a standard mechanism to verify if a target agent URI (`agent:///`) is cryptographically signed before executing tasks. This allows untrusted agent calls and unverified prompt injections to compromise multi-agent execution loops.

### Describe the Solution You'd Like
Introduce `CreduentGoogleADKPlugin` — a local Ed25519 + JCS RFC 8785 cryptographic verification plugin for agent URIs in under 5ms, with no network calls required.

### Impact on your work
Critical for securing enterprise multi-agent delegation chains. Implementation and test suite are ready to submit as a Pull Request.

### Willingness to contribute
Yes

---

## 🟡 Recommended Information

### Describe Alternatives You've Considered
1. Manual HTTP verification before sub-agent calls — adds latency.
2. Custom wrapper functions — no standard schema validation across frameworks.

### Proposed API / Implementation
```python
from creduent.integrations.google_adk import CreduentGoogleADKPlugin

plugin = CreduentGoogleADKPlugin(
agent_uri="agent://assistant.dev/agent",
timeout=10
)
```

### Additional Context
- Python SDK: `pip install creduent` / `pip install google-adk`
- Open-Source Protocol: https://github.com/idevsec/creduent