Add support for ElevenLabs TTS

[lzell]

How to use ElevenLabs for text-to-speech

    import AIProxy

    let elevenLabsService = AIProxy.elevenLabsService(
        partialKey: "partial-key-from-your-developer-dashboard",
        serviceURL: "service-url-from-your-developer-dashboard"
    )
    do {
        let body = ElevenLabsTTSRequestBody(
            text: "Hello world"
        )
        let mpegData = try await elevenLabsService.ttsRequest(
            voiceID: "EXAVITQu4vr4xnSDxMaL",
            body: body
        )

        // Do not use a local `let` or `var` for AVAudioPlayer.
        // You need the lifecycle of the player to live beyond the scope of this function.
        // Instead, use file scope or set the player as a member of a reference type with long life.
        // For example, at the top of this file you may define:
        //
        //   fileprivate var audioPlayer: AVAudioPlayer? = nil
        //
        // And then use the code below to play the TTS result:
        audioPlayer = try AVAudioPlayer(data: mpegData)
        audioPlayer?.prepareToPlay()
        audioPlayer?.play()
    }  catch AIProxyError.unsuccessfulRequest(let statusCode, let responseBody) {
        print("Received non-200 status code: \(statusCode) with response body: \(responseBody)")
    } catch {
        print("Could not create ElevenLabs TTS audio: \(error.localizedDescription)")
    }

• See the full range of TTS controls by viewing ElevenLabsTTSRequestBody.swift.
• See https://api.elevenlabs.io/v1/voices for the IDs that you can pass to voiceID.