Allow migrations configuration to be automatically be picked up from a cloned repo

[ajcvickers]

I often want to use migrations after cloning a GitHub repo or downloading a VS solution in some other way. To do this I have to figure out which project is the startup, which project contains migrations, and possibly other settings. Instead I want that information to be carried with the VS solution. This would allow me to run, for example, dotnet ef migrations add One at the solution level and have it use the appropriate projects and other settings.

This could be done in one of several ways:

• A config file at the solution level that EF reads
• Support for generating and using "response files" automatically
• Support for generating and using a well-known script automatically

This will also require documentation to promote this as a well-known pattern that becomes commonly used.

[Saibamen]

Related: #21106?

[bricelam]

A possible implementation: #22203

[bricelam]

Just to publicly state my stance on this, I don't think we'll ever be able to provide something that's more discoverable or simple than a script file can provide.

For example, by creating a file named ef.cmd (or an equivalent .sh file on macOS and Linux) in the root of your project with contents like the following:

@dotnet ef --project MyApp.Data --startup-project MyApp %*

And using commands like the following:

ef migration add MyMigration
ef database update

Or if you have multiple contexts or projects, use response files. For example create a file named identity.rsp with the following contents.

--context=IdentityContext
--project=MyApp.Data
--startup-project=MyApp

And use it like this:

dotnet ef database update @identity.rsp