SQLite RevEng: Include computed columns

[bricelam]

Note, we can't get their actual SQL without parsing the entire CREATE TABLE statement, so we just generate HasComputedColumnSql without any arguments to at least make them usable.

Fixes #21557

[bricelam]

Design meeting notes:

• Make HasDefaultValue() throw in Migrations and EnsureCreated (breaking change)
• Add HasDefaultValueSql() that behaves the same as HasDefaultValue()
• Make HasDefaultValue(null) remove the DEFAULT constraint (breaking change)
• Double-check that HasDefaultValueSql(null) removes the DEFAULT constraint
• Add HasComputedColumnSql()
  • Throw in Migrations and EnsureCreated
  • HasComputedColumnSql("") should continue to throw
• Handle HasDefaultValue(), HasDefaultValueSql(), and HasComputedColumnSql() in the model snapshot
• Handle HasDefaultValueSql() and HasComputedColumnSql() in RevEng