feat(npm): add backend to fetch data from private registries

[christoph-jerolimov]

Hey, I just made a Pull Request!

FYI @karthikjeeyar

WIP PR:

• Add backend plugin to fetch npm packages later also from non-npmjs.com and private registries (feature not added yet)
• A common plugin to share annotations, utils (isNpmPackage) and later the APIs to call non-npmjs.com and esp. private registries

I welcome any kind of early feedback.

Next steps:

• Add support for non-npmjs.com registries
• Add support for private registries
• Add support for multiple repositories (extend config.d.ts)
• Make backend optional for public npmjs.com packages?
• Add techdocs (Npmjs, GitHub packages)
• Update READMEs
• Check/extend unit tests
• Check/extend dev pages (frontend and backend)

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Thanks for the contribution!
All commits need to be DCO signed before they are reviewed. Please refer to the the DCO section in CONTRIBUTING.md or the DCO status for more info.

[backstage-goalie]

Missing Changesets

The following package(s) are changed by this PR but do not have a changeset:

• @backstage-community/plugin-npm-backend
• @backstage-community/plugin-npm

See CONTRIBUTING.md for more information about how to add changesets.

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
backend	workspaces/npm/packages/backend	none	v0.0.0
@backstage-community/plugin-npm-backend	workspaces/npm/plugins/npm-backend	none	v0.1.0
@backstage-community/plugin-npm-common	workspaces/npm/plugins/npm-common	none	v0.1.0
@backstage-community/plugin-npm	workspaces/npm/plugins/npm	none	v1.0.2