scaffolding: computed column does not include `HasComputedColumnSql`.

[olavloite]

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:

entity.Property(e => e.FullName)
    .IsRequired()
    .HasMaxLength(400)
    .ValueGeneratedOnAddOrUpdate();

Excepted:

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