Add support for database views

[matthewmcgarvey]

Fixes #453

To declare a view, instead of using the table macro, there is now a view macro. There are a few differences between the two:

• There are no default columns. If your view has a primary key or timestamps, they must be defined explicitly
• No Model::SaveOperation is defined
• If the view does not have a primary key, some methods don't work or will be missing
  • Avram::Model#reload raises a compilation error if invoked without a primary key
  • Model::BaseQuery no longer includes Avram::PrimaryKeyQueryable which has methods like #find that require a primary key

No functionality around materialized views is included with this change.

[matthewmcgarvey]

🚫 SchemaEnforcer fails. All columns for views are nilable. We'll have to be able to configure that check

[matthewmcgarvey]

This requires #556 to be merged first

[jwoertink]

I think the main thing will be to see how this works inside Lucky itself. But lets get it out there and give it a spin!