pack should support pack extension package

[natalieparellano]

Description

This is a sub-task of #1470

Just like we make buildpack packages, we should be able to make extension packages

Proposed solution

The command should work just like pack buildpack package, but with extensions:

Usage:
  pack buildpack package <name> --config <config-path> [flags]

Examples:
pack buildpack package my-buildpack --config ./package.toml
pack buildpack package my-buildpack.cnb --config ./package.toml --f file

Flags:
  -r, --buildpack-registry string   Buildpack Registry name
  -c, --config string               Path to package TOML config
  -f, --format string               Format to save package as ("image" or "file")
  -h, --help                        Help for 'package'
  -p, --path string                 Path to the Buildpack that needs to be packaged
      --publish                     Publish to registry (applies to "--format=image" only)
      --pull-policy string          Pull policy to use. Accepted values are always, never, and if-not-present. The default is always

Additional context

• This feature should be documented somewhere

[itsdarshankumar]

assign it please @natalieparellano