Prototype for simplifying Configuration in ClientCore

[alzimmermsft]

Description

PR prototyping a simplified Configuration story for ClientCore.

This removes much of the code around Configuration and greatly simplifies the story to:

A Configuration is comprised of zero or more ConfigurationSources that load configurations from their implemented location. ConfigurationSources supplied to Configuration are queried in the order they're given until one returns a non-null value.

Two ConfigurationSources, EnvironmentVariableConfigurationSource and SystemPropertiesConfigurationSource which pull values from System.getenv(String) and System.getProperties(String) respectively. Configuration.getGlobalConfiguration() uses both with preferring system properties over environment variables. Configuration.none() is added which is a Configuration with no sources, therefore always returning null.

Configuration APIs have been simplified to just get(String name) and get(String name, String... aliases) for supporting single named configurations and configurations that have additional aliases we wish to check.

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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