feat: add package-source-overrides dev container feature

[rosstaco]

Summary

Adds a new dev container Feature, package-source-overrides, for environments that cannot use public package feeds. It points npm, pnpm, pip, and NuGet at internal override sources (a pull-through proxy such as Artifactory, Nexus, or Azure Artifacts) instead of the public registries.

Configuration is written as early as possible in the container lifecycle and at root-readable locations so that other Features installed in the same container also resolve their packages through the override sources during their own installation.

What it does

• Writes standard config at system and per-user scope (root + remote user):

  Ecosystem	System	User
  npm / pnpm	/etc/npmrc (via NPM_CONFIG_GLOBALCONFIG)	~/.npmrc
  pip	/etc/pip.conf	~/.config/pip/pip.conf
  NuGet	/etc/opt/NuGet/Config/NuGet.Config	~/.nuget/NuGet/NuGet.Config

• Replace policy — public feeds are cleared (registry=, index-url, NuGet <clear/>), not augmented.
• No auth (anonymous proxies), matching the intended scope.
• strictSsl: false emits npm strict-ssl=false, pip trusted-host, and NuGet allowInsecureConnections for self-signed/HTTP proxies.
• Providing no URLs makes the Feature a safe no-op.

Lifecycle / ordering

A Feature can't force itself ahead of arbitrary third-party Features, so it declares no installsAfter (sorts as early as possible) and writes root-readable config that later Features' build-time (root) package installs pick up automatically. NOTES.md documents listing it first in overrideFeatureInstallOrder for a hard guarantee.

Options

npmRegistry, pipIndexUrl, nugetSource, nugetSourceName (default override), scope (system|user|both, default both), strictSsl (default true).

Testing

• shellcheck clean on install.sh and all test scripts.
• devcontainer features test:
  • Autogenerated no-op test passes on mcr.microsoft.com/devcontainers/base:ubuntu and debian:latest.
  • Scenarios npm, pip, nuget, all-insecure, scope-user all pass (24 checks).
• Wired into the justfile (build recipe, build-all, clean) and both CI matrices in .github/workflows/test.yaml; added a section to the root README.md.

Note: the branch was renamed from feat/package-feeds to feat/package-source-overrides to match the final feature id.