Design: Allow script/update etc. to run when context is not in target project, but can be found in services

[rowanmiller]

Listing DbContext's will return contexts that are registered in services, even if they reside in another project. Some commands (such as adding a migration) require the target project, since they generate code files, but others only need a context instance and could run by getting the context from services.

Perhaps we could also consider making this work if there is an IDbContextFactory in the target project.

This will help ASP.NET Core publishing.

cc @vijayrkn @mlorbetske

[ajcvickers]

@Eilon And this one is "patch proposed" as of this afternoon. Guess we have some conversions to have. 😺

[ajcvickers]

@bricelam Just from reading this it isn't clear why this should go into the patch. Can you explain some more?

[bricelam]

This was an ask from the Scaffolding team. Issue aspnet/Tooling#898 describes the end-to-end scenario that this fix enables.

[Eilon]

This patch fix is approved. Please follow the normal code review / pull request process and make sure you make the change in the correct branch.