Allow client to choose the request environment

[luizgustavo09]

Add support for customizable auth environments (production / sandbox)

Summary

A 3P developer requested the ability to target different Uber auth environments from the iOS SDK.

Previously the SDK hardcoded https://auth.uber.com across all auth flows with no way for 3Ps to redirect to sandbox.

• Adds a new UberEnvironment enum to UberCore with .production and .sandbox cases, each mapping to its base URL (auth.uber.com / sandbox-login.uber.com)
• Threads environment through AuthorizationCodeAuthProvider's public initializer (defaults to .production — fully backwards-compatible)
• Example app updated with an Environment picker so the flow can be tested end-to-end against sandbox

Usage

// Production (default — no change for existing integrations)
let provider = AuthorizationCodeAuthProvider()

// Sandbox
let provider = AuthorizationCodeAuthProvider(environment: .sandbox)

Test plan

• New unit tests verify production URLs contain auth.uber.com and sandbox URLs contain sandbox-login.uber.com for both in-app and native login flows
• All existing tests pass
• Manually tested in the example app using the new Environment picker

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[mohssenfathi]

Can we just pass this through to authProvider?