feat: Adds Environment Secrets & Variables

[drewid]

…Added files and content to Clients, Helpers, Models and Tests. Corrected comments on related files. Resolves Issue # 2752

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Before the change?

Functionality to use Environment Secrets & Variables did not exist.

After the change?

Added the ability to get, create and updated Environment Secrets & Variables.

Pull request checklist

• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been reviewed and added / updated if needed (for bug fixes / features) - not present in general

Does this introduce a breaking change?

• [ ] Yes
• [X] No

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[drewid]

Modeled the changes directly after the already implemented repository secrets and variables.