Scaffold PostgreSQL extensions

[roji]

After redoing the extension migrations as per #101, look into scaffolding PostgreSQL migrations from an existing database.

[roji]

Blocked on dotnet/efcore#6561

[smitpatel]

This can be unblocked once PostgreSQL aligns with EF Core 2.0.0-preview2

[roji]

Thanks @smitpatel, can you point me to an issue/PR/code sample?

[smitpatel]

Issue: dotnet/efcore#7004
PR: dotnet/efcore#8680
It was very large change but if scope it down then this commit introduce hooks for this task dotnet/efcore@32434d2
Look for SqlServer Clustered index, how the information is propagated from annotation on DatabaseModel to generated fluent API.

[roji]

This is done, thanks for the guidance @smitpatel.

Note dotnet/efcore#9121 about a limitation of the scaffolding code generation mechanism.