feat(product): Add 'product' commands for each Fastly product.

[kpfleming]

Adds 'enable', 'disable', and 'status' commands for each product previously supported by the 'products' command (which is now deprecated).

Also adds 'configure' commands for products which support configuration (DDoS Protection and Next-Gen WAF).

[kpfleming]

@cee-dub If you're interested, this could use a second 'structural' review; much like go-fastly, this now has an internal/productcore package which uses generics to provide the base implementations for all of the commands in fastly product <name> .... It's complete for all products except NGWAF, which requires an extra enablement parameter, and then it will get extended to support the configuration commands too.