refactor(serverless-api): update user-agent string for better debugging

[dkundel]

This PR standardizes the format of our User-Agent string in @twilio-labs/serverless-api and provide the ability to pass additional information from upstream packages to be included. It follows the agreed pattern of:

<core-api-lib>/<core-api-lib-version> (<os-name> <os-arch>) <extensions>

Where <extensions> are being added space separated following:

<library>/<library-version>

or

<tag>

In this case the TwilioServerlessApiClient offers to pass in userAgentExtensions as an array to specify upstream libraries who use it to add their library name and version.

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[philnash]

Mostly looks good, added some ideas about simplifying a couple of bits.

[dkundel]

I want to add at least one nock test specifically for the user-agent string before merging this but it's ready for review.

[philnash]

Looks good to me now!