Method to get EntityType properties ordered by key members first

[bricelam]

There are a few different places in the code where we want to get a list of an entity type's properties with the key members first. These currently include:

• Migrations
  • CreateTable calls
  • Model snapshot
• Reverse engineer

We should have one place to do this. For example, an extension method on IEntityType.

[bricelam]

See also this discussion.

[AndriySvyryd]

The order should be configurable in the model. The default order would be set by a convention that the providers can replace.

[mikary]

Properties are now ordered with properties that are part of the primary key first (in the order specified on the key for composite keys) in PR #1896