Introduce item subcommand for code indexer types

[calebkiage]

Currently, the API endpoints /users and /users/{user-id} both have their executors under the command mgc users. The difference is that the GET endpoints differ a little. The former generates the command mgc users list while the latter generates the command mgc users get --user-id <userId>. This works fine for most endpoints, but there's a problem with some rare endpoints that have repeated path segments e.g. /sites vs /sites/{site-id}/sites. In this case, there's a conflict with the commands mgc sites list and mgc sites list --site-id <siteId>. Adding the item subcommand for endpoints that have {xxx-id} should fix the issue.
/sites will generate the command mgc sites list while /sites/{site-id}/sites will generate the command mgc sites item list --user-id <userId>