Download & upload IPNS records through API / CLI

[cornwarecjp]

Checklist

• [X] My issue is specific & actionable.
• [X] I am not suggesting a protocol enhancement.
• [X] I have searched on the issue tracker for my issue.

Description

I would like to have API / RPC functions for downloading / uploading IPNS records. This should include both our own records and third-party records. The reason for this is that I have application-level knowledge of who might have these records and who might be interested in them, so exchanging records directly (whenever two parties are online simultaneously) is a fast and efficient shortcut for my application. Being able to also exchange them through the IPFS network is a nice extra, for redundancy / scalability.

When uploading an IPNS record to Kubo, it's important to know what happens if Kubo already knows a more recent record for that name. I'd say, by default, it should return an error and keep the more recent record. For my application, that behavior will do fine.

When uploading an IPNS record to Kubo, one might want to specify whether it should be pinned (as soon as IPNS pinning is implemented; this feature is already requested, e.g. in #1958 ). I'll add some pinning-specific thoughts to #1958.