Add option for specifying stored/virtual on computed columns

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Closes #6682

Cross-database situation

Database	Stored name	Virtual name	Default	Docs
SQL Server	PERSISTED	<empty>	<empty> (virtual)	https://docs.microsoft.com/en-us/sql/relational-databases/tables/
Sqlite	STORED	VIRTUAL	VIRTUAL	https://www.sqlite.org/gencol.html
PostgreSQL	STORED	VIRTUAL (not yet implemented)	None, STORED must currently be specified	https://www.postgresql.org/docs/12/ddl-generated-columns.html
MySQL	STORED	VIRTUAL	VIRTUAL	https://dev.mysql.com/doc/refman/5.7/en/create-table-generated-columns.html

Notes

• tl;dr everyone calls it STORED except for SQL Server (surprise) which calls it PERSISTED. So I chose STORED as our naming.
• This currently introduces a new ComputedColumnIsStored annotation alongside ComputedColumnSql. @AndriySvyryd I know you prefer to merge annotations where possible for perf, but I'm hesitant to encode both SQL and other info in the same annotation string. We can change the annotation format to <IsStored>,<SQL> and identify old annotations not starting with true/false and a comma, but since SQL is arbitrary SQL it seems a bit dangerous.
• If we keep two annotations, what should be the behavior of HasComputedColumnSql accepting IConventionPropertyBuilder? Should we have two separate ones - one for the SQL annotation and another for the IsStored - or one which checks both and only updates if both are settable? Or each one if it's settable?
• Note that this introduces a binary break by adding a default argument to HasComputedColumnSql (but recompiling is OK). If we want to avoid this we can add an overload instead.
• Instead of a stored bool we could have a ComputedColumnType enum (Stored/Virtual), but this seems needless verbose/heavy.

[AndriySvyryd]

For IConventionPropertyBuilder we should have two separate methods

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For IConventionPropertyBuilder we should have two separate methods

Thanks, done (HasComputedColumnIsStored is somewhat weird naming but consistent etc.)

[AndriySvyryd]

IsStoredComputedColumn would be better IMO

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Rebased on latest master (resolving conflicts with #20602), squashed and renamed HasComputedColumnIsStored->IsStoredComputedColumn

[AndriySvyryd]

Also add validation for properties mapped to the same column

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Thanks @AndriySvyryd!