Keep ColumnType up-to-date while model is being built

[ajcvickers]

In #8605, RelationalTypeMapping objects are bound to properties as part of the final model building step before the model is used by the runtime. This means that ColumnType is not available in OnModelCreating. We could instead keep it updated as the model is being built by implementing conventions that know when a property is added, or when any facet of that property changes that might impact the mapping. This is a lot of conventions, not all of which we have.

[ajcvickers]

If we do this, then we will do it as part of the read-only model--see #8258.

[AndriySvyryd]

Or as a default value convention #9329