Add a way to change the location of function templates

[dkundel]

In order to be more friendly to contributors and allow them to test their templates before submitting them to github.com/twilio-labs/function-templates. It would also allow us to verify that they are working before merging the PR.

Since this is not a common feature (for now), I would propose to do this as an environment variable along the lines of TWILIO_FUNCTION_TEMPLATES_ENDPOINT that can be set to override the actual variables used in data.ts: https://github.com/twilio-labs/twilio-run/blob/8fbe2e1867bafa840afab774994b46c71752f337/src/templating/data.ts#L8-L11