cmd: support multiple beacon nodes

[corverroos]

Adds support for multiple beacon nodes via eth2 multi client. Adds the --beacon-node-endpoints flags and deprecates --beacon-node-endpoint flag. Updates validatorapi proxy to use active eth2client address.

category: feature
ticket: #552

[codecov]

Codecov Report

Merging #830 (f0a9b6a) into main (abea4c0) will increase coverage by 0.73%.
The diff coverage is 52.50%.

@@            Coverage Diff             @@
##             main     #830      +/-   ##
==========================================
+ Coverage   54.10%   54.84%   +0.73%     
==========================================
  Files         109      111       +2     
  Lines       10925    11425     +500     
==========================================
+ Hits         5911     6266     +355     
- Misses       4147     4256     +109     
- Partials      867      903      +36     

Impacted Files	Coverage Δ
testutil/beaconmock/beaconmock.go	64.19% <ø> (-1.63%)	⬇️
app/app.go	55.25% <31.25%> (+0.19%)	⬆️
core/validatorapi/router.go	65.68% <50.00%> (-1.19%)	⬇️
cmd/run.go	85.29% <66.66%> (-14.71%)	⬇️
testutil/beaconmock/options.go	43.91% <0.00%> (-4.68%)	⬇️
core/deadline.go	64.86% <0.00%> (-3.14%)	⬇️
core/fetcher/fetcher.go	65.07% <0.00%> (-0.52%)	⬇️
cluster/confighash.go	100.00% <0.00%> (ø)
... and 9 more

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[dB2510]

LGTM