Add InvocationContext.GetHost extension method

[fredrikhr]

Adds an extension method GetHost to InvocationContext, so that Command-line Middleware that runs after a call to UseHost() can access the created IHost instance easily.

It does technically not depend on using UseHost() since it simply news up a ModelBinder<IHost> and passes the Invocation BindingContext to it. Therefore, anyone adding IHost to the BindingContext service provider will be able to use this extension to access their IHost.

fixes #974

ref.: #974 (comment)

[jonsequitur]

Could you add a test that illustrates and verifies the use case?

[fredrikhr]

@jonsequitur Test added

[jonsequitur]

@fredrikhr Could you rebase this onto main?

[fredrikhr]

@jonsequitur done, ready for merge