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 #1724
• Related to similar ServiceControl feature request Add external configuration providers ServiceControl#4522

This is the same feature request as Particular/ServiceControl#4522, and given the higher complexity of ServiceControl it makes more sense to defer to that issue for feature description and discussion.

One way that ServicePulse is (at least currently) different is that only the ServicePulse container has any configuration, while the Windows-installed version does not. Therefore the documentation of the environment variables is on the Running ServicePulse in containers in the documentation.

Additional Context

No response