Remove dependency on IConfigurationRoot in external service URL resolution

[idg10]

If a service enables external URL resolution with a call such as this:

services.AddExternalServices(
    "ExternalServices",
    externalServices =>
        externalServices.AddExternalServiceWithEmbeddedDefinition<OperationsStatusOpenApiService>("OperationsStatus"));

this introduces a service dependency on IConfigurationRoot. The AddExternalServices method takes a configuration section name, and retrieves it directly from IConfigurationRoot. The OpenApiExternalServices class that it passes this to then goes on to read configuration values dynamically from the configuration section in question based on the argument to AddExternalServiceWithEmbeddedDefinition.

So with the example above, it will look for an ExternalServices section with an OperationsStatus value. So we'd need a setting of the form ExternalServices:OperationsStatus

We've been trying to remove direct dependencies on IConfiguration or IConfigurationRoot from Corvus and Menes, partly because it can make it hard to discover what the expected configuration entries are, and also because it introduces complications with tests by requiring test code to supply an IConfiguration instead of just passing in the values of interest. We should make it possible to use this without needing IConfiguration either.