Clap should be an optional feature

[adamchalmers]

twilio-api derives clap::ValueEnum here, which means any consumers of twilio-api need to compile clap. Clap is a really big crate, and consumers shouldn't be forced to compile clap unless they actually run a CLI.

Suggestion: twilio-api should have an optional Cargo feature for clap. Consumers who want CLI support can opt into it. Consumers (like api-deux) that don't need it, won't enable it, and won't have to compile clap.