Register PG extensions in the model by convention

[roji]

By scanning property types in the model.

@AndriySvyryd, this adds two ModelFinalizing conventions which scan all properties, get their type mappings, and for certain supported types that are known to require PostgreSQL extensions, adds those to the model. Up to now users have had to add these explicitly. Any intuition on whether this is problematic perf-wise?

Closes #2137

[AndriySvyryd]

Any reason not to add these extensions always? I assume there's a high chance they would be needed when the plugin is used.

[roji]

For the NetTopologySuite plugin you're right - I'll do that at the minimum to avoid yet another pass over the model.

The problem is with the convention in the provider itself, for types which rare (hstore, ltree, lquery...); I definitely don't want to add those fringe extensions for everyone...

[AndriySvyryd]

Once dotnet/efcore#9329 is implemented this shouldn't be too bad perf-wise