Support removal of keys from IConfigurationRoot

[flcdrg]

As currently implemented, Microsoft.Extensions.Configuration appears to only support an 'additive' model. Providers can add additional (or overwrite existing) keys with new values, but there's no way to remove a key.

I would like the ability to be able to remove a key from the IConfigurationRoot (or IConfiguration - whichever makes more sense)

[maryamariyan]

@flcdrg thanks for the issue. The best next step on this would be to prepare the API proposal, usage, etc. in the issue page here (following https://github.com/dotnet/runtime/blob/master/docs/project/api-review-process.md).

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