Custom Provider

[derekmckinnon]

This PR removes Amazon.Extensions.Configuration.SystemsManager in favour of a custom provider that can load JSON and YAML configurations from AppConfig.

This functionality will be released first as a prerelease package update.

• Remove obsolete code
• Add custom configuration provider and source
• Update extension method to use new configuration source
• Remove dependency on Amazon.Extensions.Configuration.SystemsManager
• Bump test dependencies
• Add ChangeToken integration
• Use application/x-yaml for YAML configurations coming from AppConfig
• Update dependencies
• Add tests for configuration provider
• Update workflows to resolve publishing issues
• Test cleanup
• Update README and metadata