Review use of IConfigurationRoot

[jamesbroome]

Proposal to change AddExternalServices so that I takes a strongly typed ExternalServicesOptions class instead of having to resolve IConfigurationRoot using a config section name.

New options class just needs BaseUrl as initial property for now, but can be expanded in the future if needed.

See: https://github.com/menes-dotnet/Menes/blob/5f653f387836a8e11ca015a0fc52cd1b2e36d56f/Solutions/Menes.Hosting/Microsoft/Extensions/DependencyInjection/OpenApiHostingServiceCollectionExtensions.cs

Only known use of this method is in Marain.Operations:

https://github.com/marain-dotnet/Marain.Operations/blob/3b86ab30feb0ae3c0ed429bb7f766fd219ee2f78/Solutions/Marain.Operations.OpenApi/Marain/Operations/OpenApi/OperationsStatusOpenApiService.cs