Support `ON DELETE SET DEFAULT`

[Atulin]

What problem are you trying to solve?

I ran across this limitation when trying to automagically move content of users that delete their accounts to a Deleted User kind of an account. Most databases support it fairly easily, say

CREATE TABLE posts (
    id SERIAL PRIMARY KEY,
    content TEXT,
    user_id INTEGER DEFAULT 1 REFERENCES users(id) ON DELETE SET DEFAULT
);

Unfortunately, EF Core only supports cascade, set null, no action, restrict, and client set null.

Describe the solution you'd like

Add .SetDetault to the DeleteBehavior enum and generate appropriate SQL.

[MihuBot]

I'm a bot. Here are possible related and/or duplicate issues (I may be wrong):

• https://github.com/dotnet/efcore/issues/7530
• https://github.com/dotnet/efcore/issues/12661

[roji]

Makes sense to me. Not sure what additional work this involves beyond adding the enum and corresponding change in MigrationsSqlGenerator (change tracking behavior, @AndriySvyryd?). Putting in the backlog for now.