Add Go facilitator client support for supported payments and discovery resources

[winor30]

• Extend the Go facilitator client to expose Supported and ListDiscoveryResources operations and share auth header handling alongside query encoding for discovery requests.
• Introduce typed models for supported payment kinds and discovery responses while expanding the facilitator auth helper to sign GET requests with method-specific tokens.

[cb-heimdall]

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[vercel[bot]]

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