Add support for facilitator API keys

[zachalam]

Description

Add support for facilitator API keys in x402proxy. The proxy currently connects to facilitators using configurable URLs, but does not yet handle authenticated requests requiring API keys.

Use Case

Some facilitators require an API key for access. Supporting facilitator API keys will allow x402proxy to work with authenticated facilitator endpoints and expand compatibility across environments.

Proposed Solution

Extend the configuration to include both the facilitator URL and an associated API key. Load the API key from an environment variable (e.g., FACILITATOR_API_KEY), and ensure that requests sent through x402proxy include the appropriate authentication header or query parameter as needed.

Alternatives Considered

Continue using open facilitator endpoints without authentication, but this approach limits deployment options where API access control is required.

Additional Context

x402proxy already integrates the x402-proxy package, so the implementation changes should be minimal. The update primarily involves adding environment variable handling and authentication header injection for facilitator-bound requests.