cmd: write expanded form of enr in charon enr

[xenowits]

• charon create enr now errors when charon-enr-private-key already exists.
• charon enr now also outputssecp256k1 pubkey, signature and seq in addition to the enr when the --verbose flag is provided.

category: refactor
ticket: #893

[codecov]

Codecov Report

Merging #913 (17a79e4) into main (a144a15) will decrease coverage by 0.27%.
The diff coverage is 5.12%.

@@            Coverage Diff             @@
##             main     #913      +/-   ##
==========================================
- Coverage   54.82%   54.55%   -0.28%     
==========================================
  Files         114      115       +1     
  Lines       12342    12497     +155     
==========================================
+ Hits         6767     6818      +51     
- Misses       4599     4678      +79     
- Partials      976     1001      +25     

Impacted Files	Coverage Δ
cmd/enr.go	6.34% <2.85%> (-5.42%)	⬇️
cmd/createenr.go	82.35% <25.00%> (-4.89%)	⬇️
eth2util/signing/signing.go	38.52% <0.00%> (ø)
core/dutydb/memory.go	71.91% <0.00%> (+1.27%)	⬆️

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