Nexus integration - pypi, npm

[andrew-jameson]

Summary of Changes

Pull request closes #3171

How to Test

TBD

Demo GIF(s) and screenshots for testing procedure

Deliverables

More details on how deliverables herein are assessed included here.

Deliverable 1: Accepted Features

Checklist of ACs:

• [ ] Pipeline builds don't reach out to the internet by default
• [ ] Pipeline builds will fail-through to internet repositories if Nexus connection fails
• [ ] Pipeline build times are reduced
• [ ] lfrohlich and/or adpennington confirmed that ACs are met.

Deliverable 2: Tested Code

• Are all areas of code introduced in this PR meaningfully tested?
  • [ ] If this PR introduces backend code changes, are they meaningfully tested?
  • [ ] If this PR introduces frontend code changes, are they meaningfully tested?
• Are code coverage minimums met?
  • [ ] Frontend coverage: [insert coverage %] (see CodeCov Report comment in PR)
  • [ ] Backend coverage: [insert coverage %] (see CodeCov Report comment in PR)

Deliverable 3: Properly Styled Code

• [ ] Are backend code style checks passing on CircleCI?
• [ ] Are frontend code style checks passing on CircleCI?
• [ ] Are code maintainability principles being followed?

Deliverable 4: Accessible

• [ ] Does this PR complete the epic?
• [ ] Are links included to any other gov-approved PRs associated with epic?
• [ ] Does PR include documentation for Raft's a11y review?
• [ ] Did automated and manual testing with iamjolly and ttran-hub using Accessibility Insights reveal any errors introduced in this PR?

Deliverable 5: Deployed

• [ ] Was the code successfully deployed via automated CircleCI process to development on Cloud.gov?

Deliverable 6: Documented

• [ ] Does this PR provide background for why coding decisions were made?
• [ ] If this PR introduces backend code, is that code easy to understand and sufficiently documented, both inline and overall?
• [ ] If this PR introduces frontend code, is that code easy to understand and sufficiently documented, both inline and overall?
• [ ] If this PR introduces dependencies, are their licenses documented?
• [ ] Can reviewer explain and take ownership of these elements presented in this code review?

Deliverable 7: Secure

• [ ] Does the OWASP Scan pass on CircleCI?
• [ ] Do manual code review and manual testing detect any new security issues?
• [ ] If new issues detected, is investigation and/or remediation plan documented?

Deliverable 8: User Research

Research product(s) clearly articulate(s):

• [ ] the purpose of the research
• [ ] methods used to conduct the research
• [ ] who participated in the research
• [ ] what was tested and how
• [ ] impact of research on TDP
• [ ] (if applicable) final design mockups produced for TDP development