Update Endpoint configuration

[renal128]

Summary

This is a step towards getting rid of global SDK config.

• Add Endpoints on ConversationConfig: an apiBase for HTTP calls (the conversation token URL derives from it, there will be more endpoints using it later), plus explicit voice and text WebSocket URLs
• Wire endpoints through Dependencies → TokenService / WebSocketConnectionManager / WebRTCConnectionManager
• Delete ConnectionConstants and remove apiEndpoint / websocketUrl from global ElevenLabs.Configuration (logging configure remains)
• Simplify the API of TokenService - return just the token instead of a unnecessary wrapper.

Test plan

• swift test

Stacked on #218.

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Note

Medium Risk
Public API breaks for apps that customized endpoints via global ElevenLabs.Configuration; connection and token URLs now depend on per-session config, though production defaults match prior behavior.

Overview
Moves network targeting off global SDK config onto ConversationConfig.endpoints via a new Endpoints type (apiBase, voiceWebSocket, textWebSocket, defaulting to production). Token fetch, LiveKit voice connect, and text WebSocket URLs now read from that config through Dependencies, TokenService, WebRTCConnectionManager, and WebSocketConnectionManager.

ElevenLabs.Configuration no longer exposes apiEndpoint / websocketUrl (logging configure remains). ConnectionConstants is removed; the conversation token HTTP URL is derived from apiBase.

TokenService / TokenServicing are simplified: fetchToken(for:) returns a String instead of a ConnectionDetails bundle with server URL and empty room fields. ConversationClient builds Dependencies(endpoints: config.endpoints) when starting a session.

README adds a Custom Endpoints example; tests cover defaults, overrides, token URL derivation, and preserving existing query items on text WebSocket URLs.

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