Subcommand support in Automatic Extension Resolution

[andrewslotin]

In k6 v1.5.0 we added a new type of extensions that can introduce custom subcommands in k6. To add a custom subcommand users need to follow the xk6 approach and compile a custom k6 binary with extension module.

As a next step we should provide the experience we already have for protocol extensions, where k6 automatically detects unmet dependencies and installs them using Automatic Extension Resolution