Design: Public API to instantiate DbContext classes

[bricelam]

Scaffolding is now using DbContextOperations.CreateContext() to instantiate contexts at design-time; however, this is an internal API.

Model visualization may require the same functionality.

We should consider adding public API with this functionality.

cc @prafullbhosale

[bricelam]

If we do, we should add it to 1.0.3 as well.

[bricelam]

@Eilon @ajcvickers Do we want to try and get this into 2.0.0-preview1 too? The work for Program.BuildWebHost (#8164) has broken the internal APIs Scaffolding was using...

[bricelam]

(and possibly 1.0.4 & 1.1.2 so Scaffolding doesn't have to condition it on version)

[ajcvickers]

@bricelam Without this, what is broken? When you say "Scaffolding" I'm not sure which "Scaffolding" that is--reverse engineering, or asp.net scaffolding?

Why would we put it in 1.0.4/1.1.2? What is broken there that this would fix?

[bricelam]

The Scaffoding repo. Anything they use a DbContext for is broken.

⭐️ I'll add back an overload to maintain compatibility in 2.0.0-preview1. ⭐️

[ajcvickers]

@bricelam What's the priority of this for 2.0?

[bricelam]

We should do it so Scaffolding doesn't have to rely on internal API. Shouldn't be much work.