Add support for aggregator API keys

[ristoalas]

This adds support for sending API keys to the subscription proxy of aggregators. It also adds a couple of exceptions for error cases.

[Copilot]

Pull Request Overview

This PR adds support for sending API keys to the subscription proxy of aggregators, enabling authentication for protected endpoints. It also introduces new exception types for handling authorization and rate limiting errors.

Key changes:

• Adds API key support to AggregatorClient with optional authentication
• Introduces UnauthorizedException and RateLimitExceededException for error handling
• Updates JsonRpcHttpTransport to handle API key headers and HTTP error responses

Reviewed Changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
TestApiKeyIntegration.java	Comprehensive test suite for API key authentication scenarios
MockAggregatorServer.java	Test mock server with API key validation and rate limiting simulation
UnauthorizedException.java	New exception for HTTP 401 unauthorized errors
RateLimitExceededException.java	New exception for HTTP 429 rate limit errors with retry information
JsonRpcHttpTransport.java	Enhanced transport layer with API key support and error handling
AggregatorClient.java	Updated client with optional API key constructor and authentication

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}