Implement value conversions that spread out over multiple columns

[AndriySvyryd]

Multi-column value conversions were initially tracked by #13947 as a way to enable value objects, but we decided to instead implement them using complex types.

This issue tracks using value conversions as an alternative to complex types.

Pros

• How properties decompose into columns is more flexible (e.g. two properties can be stored in one column or viceversa)

Cons

• Opaque to queries (i.e. individual properties participating in a multi-column value conversion can't be used in conditions or projections)

[mguinness]

From linked issue:

Another area where this could be useful is storing DateTimeOffset into two fields for databases like MySQL which have no equivalent data type.