ipfs key export pubkey only option

### Checklist

- [x] My issue is specific & actionable.
- [x] I am not suggesting a protocol enhancement.
- [x] I have searched on the [issue tracker](https://github.com/ipfs/kubo/issues?q=is%3Aissue) for my issue.

### Description

As a user I want to be able to access my public key so that I can send it to people. Further, I want to have it in ed25519 format, which I only see in three places;

```bash
$ ./ipfs key gen gh

# Output
# k51qzi5uqu5dkp219vs64qzyyif5yyh2dwo12zrlc7slwxkckpisg01k48x2yj
```

```bash
./ipfs name publish /ipfs/bafkreifmyb5wf4r7iwesg434jtkkm26qluphd22phbaahovpfxghmdldje --key gh

# Output
# Published to k51qzi5uqu5dkp219vs64qzyyif5yyh2dwo12zrlc7slwxkckpisg01k48x2yj: /ipfs/bafkreifmyb5wf4r7iwesg434jtkkm26qluphd22phbaahovpfxghmdldje

echo "data" | ./ipfs key sign --key my-new-key

# Output
# {"Key":{"Name":"my-new-key","Id":"k51qzi5uqu5dkyu7b6rdcayq3tvmfwqh90hjea80n5p9ih4p3amxar98c1r0wp"},"Signature":"ubefFmYU0dNJfF7r_pR02ulm74gCmuis3xUnbyx7JcKcZXDUBt6xj0KGIeWrNnW7-viYXAXnr5EBiXUzs2FxeDw"}
```

It should be easier to access, and then it would make the key-signing/verify procedure easier for those sharing signed data via IPFS and verifying it via IPFS.