Refactor to use annotation access pattern

[austindrenski]

I realized while working on #685 that PostgresExtension, PostgresEnum, and PostgresRange broke with (what seems like) the standard annotation access patterns used throughout EF Core.

[roji]

The overall direction is good and makes the good more clean/idiomatic. But please see notes about where the annotations belong.

[austindrenski]

@roji I took another pass at this with better naming and more specific extension methods (i.e. not on IAnnotatable).

I think it's a step in the right direction, but may still need some refinement.

[roji]

It looks close to ready, see my comment about the interfaces.

For the record I don't think there's huge value in this - it does make for a slightly nicer interface to the annotation data, but lots of code is added to support this...