New loopback-cli command: update/migrate the database

[bajtos]

As a follow-up to the discussion in #3395, we should implement a new loopback-cli command that will allow users to auto-update their databases and more importantly, to print (SQL) scripts needed to perform auto-update, so that users can run these scripts themselves.

Related reading: https://www.martinfowler.com/articles/evodb.html#AllDatabaseChangesAreMigrations

A possible implementation:

$ lb autoupdate db --dry
ALTER TABLE AccessTokens
ADD COLUMN scopes VARCHAR(255);

[shaheero]

Another column is needed if you follow the steps in the docs. i.e. if several models are extending the built-in User model.
"principalType" with type VARCHAR and length 255

[bajtos]

LoopBack version 3 is in LTS, we won't be adding any new features.