Add external configuration providers

[DavidBoike]

Describe the feature.

• More focused subset of features requested in Enable cloud configuration as a 1st class option for containers #3966
• Related to similar ServicePulse feature request Add external configuration providers ServicePulse#2100

ServiceControl can be configured via configuration file or environment variable, and since version 5.3.0 the environment variables have been documented in addition to the App config keys in the documentation for the Error instance, Audit instance, and Monitoring instance.

What is missing is the ability to configure settings from a centralized source such as Azure App Configuration, Consul, or other tools.

To use Azure App Config, for instance, a single dedicated environment variable would be necessary to configure the App Config connection string, and perhaps another for environment, and if those were present, the app would reach out to the service to download the configuration settings. These would participate with the settings readers for the App Config and EnvVar readers. (That is, unless the XML-based app.config has been replaced by configuration based on Microsoft.Extensions.Configuration.)

Additional Context

Note that it doesn't really make sense to implement this feature with also doing the same for ServicePulse.