Accounts V2: Create Wallet Non-Interactively + Add Test Coverage

[rauljordan]

What type of PR is this?

Feature

What does this PR do? Why is it needed?

This PR allows us to run prysm validator wallet-v2 create non-interactively with a pure, flag-based setup.

Remote Keymanager Wallet Creation

bazel run //validator -- wallet-v2 create \
  --wallet-dir=/tmp/wallet \
  --passwords-dir=/tmp/walletpasswords \
  --keymanager-kind=remote \
  --grpc-remote-address=host.example.com:4000 \
  --remote-signer-crt-path=/tmp/client.crt \
   --remote-signer-key-path=/tmp/client.key \
   --remote-signer-ca-crt-path=/tmp/ca.crt

Direct Keymanager Wallet Creation

bazel run //validator -- wallet-v2 create \
  --wallet-dir=/tmp/wallet \
  --passwords-dir=/tmp/walletpasswords \
  --keymanager-kind=direct

Which issues(s) does this PR fix?

Part of #6220

[codecov]

Codecov Report

Merging #6611 into master will increase coverage by 1.48%.
The diff coverage is 71.60%.

@@            Coverage Diff             @@
##           master    #6611      +/-   ##
==========================================
+ Coverage   60.07%   61.56%   +1.48%     
==========================================
  Files         323      373      +50     
  Lines       27422    29239    +1817     
==========================================
+ Hits        16473    18000    +1527     
- Misses       8733     8770      +37     
- Partials     2216     2469     +253