feat: human-readable cache keys for IPNS

[laurentsenta]

Closes #37

Use ipnsKey.String() to generate cache keys, which generate a b58 encoding instead of using non-printable characters.
This makes it practical to populate the IPFS_NS_MAP env variable with IPNS records:

export IPFS_NS_MAP="dnslink-test.example.com:/ipfs/${CID},12D3KooWQbpsnyzdBcxw6GUMbijV8WgXE4L8EtfnbcQWLfyxBKho:/ipfs/${CID}"

[lidel]

lgtm, but we need to normalize key

[lidel]

This won't be enough because of legacy mess: /ipns/id can be Base58btc of peerid (12... which is not a valid CID), or CIDv1 with libp2p-key multihash in base36 (k51..).

This screates surface for bugs, where someone tries to use CIDv1 base36 (k..) produced by ipfs key list -l and it does not work because raw peerid 12... is expected here.

I think we need something like:

import ( 
  iface "github.com/ipfs/interface-go-ipfs-core"
...
		cacheKey = iface.FormatKeyID(ipnsKey)

This way the key will be canonical cidv1+libp2p-key+base36, the same user can see in ipfs key list -l

[laurentsenta]

Got it, thanks for the review and details.

I guess since all the cache keys go through the same calls, that's "just" a problem for the user that tries to use something other than a raw peer id in IPFS_NS_MAP.

I updated the PR so that we normalize all the keys loaded from IPFS_NS_MAP with the same FormatKeyID, and added tests!

[lidel]

Thank you @laurentsenta.
Merging, so we include this in future release.