feat: Gateway Control Plane Client

[Hweinstock]

Problem

The SDK has no gateway management surface area. Customers who need to programmatically create MCP gateways, manage targets (Lambda, OpenAPI, Smithy), configure credentials (API key, OAuth2), attach policy engines, or toggle observability on gateways have no SDK path — they must use the CLI or console. The starter toolkit's GatewayClient demonstrates the full scope of what the SDK should support: CRUD, target management, name-based resolution, and status polling.

Acceptance Criteria

• Customers can create a gateway and the SDK polls until it reaches READY status
• Customers can get, list, update, and delete gateways
• Customers can create, get, list, and delete gateway targets (Lambda, OpenAPI, Smithy)
• Customers can configure credential providers (API key, OAuth2) on gateway targets
• Customers can attach/update policy engine configuration on a gateway
• Name-based lookup is supported (resolve gateway/target by name, not just ID)
• All functionality is verified via integration tests running in CI

Relevant Links

• GatewayClient
• create_mcp_gateway()
• create_mcp_gateway_target()
• delete_gateway()
• update_gateway()
• update_gateway_policy_engine()