Add NSID to network identifiers

[johanstokking]

Summary

Open source counterpart of https://github.com/TheThingsIndustries/lorawan-stack/pull/3811

This adds the NSID to network identifiers.

Changes

• Add NSID to Network Server configuration and include in uplink message's network metadata
• Use interop's NSID in Identity Server and return as part of HomeNSAns
• Fix unrelated comments

Testing

CI

Regressions

None

Notes for Reviewers

Review in https://github.com/TheThingsIndustries/lorawan-stack/pull/3811 please.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• The steps/process to test this feature are clearly explained including testing for regressions.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.