Support cleartext PEM key import/export

[lidel]

go-ipfs 0.13+ will support key import/export using cleartext PEM/PKCS8 format (ipfs/kubo#8616) which enables interop with things like openssl, for example;

Key generation outside the IPFS node:

  $ openssl genpkey -algorithm ED25519 > ed25519.pem
  $ ipfs key import test-openssl -f pem-pkcs8-cleartext ed25519.pem

Using openssl to get a PEM with public key:

  $ ipfs key export testkey --format=pem-pkcs8-cleartext -o privkey.pem
  $ openssl pkey -in privkey.pem -pubout > pubkey.pem

js-libp2p-crypto should support PEM format to enable similar level of interop.
we would like to use it in ipfs/js-ipfs#2887

Ref.

• https://github.com/libp2p/js-libp2p-crypto/tree/master/src/keys
  • note: in v0.22.x only the encrypted export using the proprietary format is supported for ed25519