Add Nexus repositories

[jemrobinson]

✅ Checklist

• You have given your pull request a meaningful title (e.g. Enable foobar integration rather than 515 foobar).
• You are targeting the appropriate branch. If you're not certain which one this is, it should be develop.
• Your branch is up-to-date with the target branch (it probably was when you started, but it may have changed since then).
• You have marked this pull request as a draft and added '[WIP]' to the title if needed (if you're not yet ready to merge).
• You have formatted your code using appropriate automated tools (for example ./tests/AutoFormat_Powershell.ps1 -TargetPath <path to file or directory> for Powershell).

⤴️ Summary

• Add Nexus proxy for PyPI and CRAN to each SRE
• Add configuration option to use "all", "pre-approved" or "none" lists of packages

🌂 Related issues

Closes #1481

🔬 Tests

Tested on a dev Pulumi deployment