scaffolding: computed column does not include `HasComputedColumnSql`.

The current scaffolding generator ensures that a valid `DbContext` that can be used is generated. If we also include the computed SQL in the meta model, EF Core will not respect the `ValueGeneratedOnAddOrUpdate` part of the meta model, and will try to insert values into the computed column. Cloud Spanner does not allow that.

Query:
`FullName STRING(400) NOT NULL AS (COALESCE(FirstName || ' ', '') || LastName) STORED,`

Actual:
```cs
entity.Property(e => e.FullName)
.IsRequired()
.HasMaxLength(400)
.ValueGeneratedOnAddOrUpdate();
```

Excepted:
```cs
entity.Property(e => e.FullName)
.IsRequired()
.HasMaxLength(400)
.ValueGeneratedOnAddOrUpdate()
.HasComputedColumnSql("(COALESCE(FirstName || ' ', '') || LastName) STORED");
```