Allow Importing One-Off, Private Keys Into Accounts-v2

[rauljordan]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

A nice feature to have is to be able to import one-off private keys into the Prysm wallet, which would help for importing keys from other client teams or from any other source in the simplest way. The added feature is:

bazel run //validator -- accounts-v2 import --import-private-key-file=privkey.txt

Where privkey.txt contains a hex string representation of the private key

0x5716c24494e1892ab4169cec93a18c69414ca1830d12f9ded2070b20e7df5c84

Running the command results in

Importing accounts, this may take a while...
Importing accounts... 100% [============================================================================================]  [0s:0s]
Successfully imported account from private key, view all accounts by running accounts-v2 list

Which issues(s) does this PR fix?

Fixes #6805

[codecov]

Codecov Report

Merging #6929 into master will increase coverage by 0.27%.
The diff coverage is 52.94%.

@@            Coverage Diff             @@
##           master    #6929      +/-   ##
==========================================
+ Coverage   61.91%   62.18%   +0.27%     
==========================================
  Files         395      393       -2     
  Lines       30914    30597     -317     
==========================================
- Hits        19139    19028     -111     
+ Misses       9050     8881     -169     
+ Partials     2725     2688      -37     

[farazdagi]

Tested locally, works great.