Set Client User Agent Property to allow comlink usage tracking

[ayyrickay]

const client = new Twilio(apiKey, apiSecret, {accountSid: '...', userAgentExtensions: ['@twilio-labs/plugin-dev-phone/1.0.0']})

const client = new Twilio(apiKey, apiSecret, {accountSid: '...', userAgentExtensions: ['@twilio-labs/plugin-dev-phone/1.0.0', 'dev-phone-headless']})

We can set this up to consume an array of tags - so there may be different modes that we want to support in the future, like Headless and Functions.

Note that this needs to be implemented with Functions as well as the extension client itself. Those should have separate tracking, because they can tell us different info (functions for Usage, extension for number of setups - interesting ratio!)

Example of implementation here: https://github.com/twilio-labs/twilio-firebase-extensions/blob/main/send-message/functions/src/utils.ts#L22-L24

[ayyrickay]

Leaving this open because we still need to implement tracking in the dev phone client's requests

[ayyrickay]

    // TODO: something like this.twilioClient.userAgentExtensions = [ARRAY_OF_HEADER_STRINGS]

Can put this in dev-phone.ts