Data structure only, submitted standalone as asked by CONTRIBUTING.md, so it can be
reviewed and accepted before the code that uses it.
A contact can either carry its own address or reuse the address of its thirdparty.
Today the choice is guessed from the content of the address fields, which cannot tell
an empty address from a deliberate "use the thirdparty address". The new column stores
the choice explicitly.
0 = use the address of the contact
1 = use the address of the linked thirdparty
NULL = legacy resolution, the value for every existing record, so an existing
alternative address stays independent from its thirdparty address.
Code side: #38118.
5847c6b687