p2p: store private keys as PKCS#8 ASN.1 DER PEM

[algorandskiy]

Summary

Before: p2p private key is stored raw in *.pem file. It is misleading since it does not contain an expected PEM content.
After: ED25519 key encoded as PKCS#8 ASN.1 DER and stored in PEM format, block type "PRIVATE KEY" in a *.key file.

Note: this implementation relies libp2p/crypto's implementation details that it uses golang's standard crypto.ed25519.PrivateKey underlying representation. It looks OK since we already have the same assumption in PeerIDChallengeSigner implementation.

Test Plan

Existing tests passed
Checked openssl understand this key

goal node -d . generate-p2pid
[Data Directory: /data]
PeerID: 12D3KooWLwzibGo6GkGVnyasbfQxxcjng3QJEAqXgMTPm7VtG9TG
Private key saved to /data/peerIDPrivKey.key

goal node -d . generate-p2pid
[Data Directory: /data]
PeerID: 12D3KooWLwzibGo6GkGVnyasbfQxxcjng3QJEAqXgMTPm7VtG9TG
Used existing key /data/peerIDPrivKey.key

openssl pkey -in peerIDPrivKey.key -text -noout
ED25519 Private-Key:
priv:
    34:10:37:ef:9e:31:38:e0:e8:89:28:b1:28:28:6f:
    80:17:48:20:ee:1e:dc:59:5f:3d:8e:16:2f:44:7e:
    ca:5a
pub:
    a5:64:f2:b2:e4:4f:b9:2d:25:74:84:61:1a:b4:71:
    5b:d1:ef:a7:81:59:2d:53:e7:5d:a6:bf:3f:18:3b:
    64:9b

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 56.18%. Comparing base (81edd96) to head (52027bd).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
network/p2p/peerID.go	50.00%	6 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6119      +/-   ##
==========================================
- Coverage   56.20%   56.18%   -0.03%     
==========================================
  Files         492      492              
  Lines       69829    69851      +22     
==========================================
- Hits        39248    39244       -4     
- Misses      27915    27924       +9     
- Partials     2666     2683      +17     

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