feat: add ability to export a key

[SgtPooki]

Users cannot currently export a key using the kubo-rpc-client API.

There are security risks here with allowing users to export a key, but kubo users can do so via command line. We were asked by a user to allow import+export of a key via UI but there is no way to export a key via the RPC currently.

Note: Depends on changes to Kubo (if this is something we even want to do)

[lidel]

The security concern remains, but we have more options now.

We could allow export over HTTP RPC API if (and only if) an API.Authorizations protection exists and applies to both /api/v0/config (so config cant be easily changed) and /api/v0/key/export.

This requires changes to Kubo (afaik key export only works in offline mode where direct filesystem access to the repo is used, and is explicitly blocked over HTTP)