Support custom config and simplify environment selection

[curtisfennerpaypay]

This PR simplifies the way that the environment (staging vs prod vs perf) is configured, and also enables custom environment configuration (Fixes #500).

The constructor for the internal class Conf is modified in a breaking way, however it is unlikely that this class was used by any consumers.

Previously, Boolean flags productionMode and perfMode could be used to pick one of three environments. Now, a string environment name ("PROD", "STAGING", or "PERF_MODE") can be used, or a custom instance of the Conf class. The old flags also remain, for backwards compatibility, but the documentation has been updated to show the new method.

New Feature Submissions:

1. Does your submission pass tests?
2. Have you lint your code locally prior to submission?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[blackduck-copilot]

Black Duck Security Report

Merging #504 into master will not change security risk.

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

No Coverage information
0.0% Duplication