Include inode in JSON output for find and ls

[dnnr]

What does this PR change? What problem does it solve?

This introduces the inode attribute to the JSON output emitted for nodes in ls and matches in find. There doesn't seem to be any discernible reason to omit the inode and it can be useful in scripting scenarios.

Was the change previously discussed in an issue or on the forum?

There was a brief discussion in the IRC channel.

Checklist

• I have read the contribution guidelines.
• I have enabled maintainer edits.
• I have added tests for all code changes.
• I have added documentation for relevant changes (in the manual).
• There's a new file in changelog/unreleased/ that describes the changes for our users (see template).
• I have run gofmt on the code in all commits.
• All commit messages are formatted in the same style as the other commits in the repo.
• I'm done! This pull request is ready for review.

[dnnr]

I'm not entirely sure if I should add any tests for this. Many of the existing attributes currently don't seem to be tested either. cmd_ls_test.go appears to cover some special cases but there's no basic case that just confirms the full list of possible fields. Should I add something like that, or would you consider it rather pointless/superfluous as it just reiterates the implementation?

[MichaelEischer]

LGTM.

Ideally, we'd extract the restic.Node to JSON conversion into a separate method (using the approach from cmd_ls.go) and add a very basic test just to make sure that the JSON output doesn't change unexpectedly. But that's out of scope for this PR.