Correct how AllValidatorsAreExited creates status request

[dv8silencer]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?
This bug fixes #7707 -- If you look at this in validator/client/validator.go

	var publicKeys [][]byte
	for _, key := range validatingKeys {
		publicKeys = append(publicKeys, key[:])
	}
	request := &ethpb.MultipleValidatorStatusRequest{
		PublicKeys: publicKeys,
	}

What I saw during debugging is that after the range-for loop, publicKeys ends up with a set of identical keys that match the last key appended. This is likely because the iterating variable (key)'s memory address is appended and since this address will last hold the last key iterated over in validatorKeys, publicKey ends up with multiple copies of the same key. If the validator associated with the last key is exited, then this function will true that all validators are exited and this leads to the described bug. To fix this I just create a copy and append that instead. There is a regression test created as well which creates a couple of sample keys and sees if the request is as expected. This test fails on current master and passes with this PR.

Which issues(s) does this PR fix?

Fixes #7707

Other notes for review

[codecov]

Codecov Report

Merging #7758 (7181cc7) into master (d4c9546) will decrease coverage by 0.01%.
The diff coverage is 52.70%.

@@            Coverage Diff             @@
##           master    #7758      +/-   ##
==========================================
- Coverage   62.15%   62.14%   -0.02%     
==========================================
  Files         429      429              
  Lines       30248    30290      +42     
==========================================
+ Hits        18802    18824      +22     
- Misses       8521     8536      +15     
- Partials     2925     2930       +5     

[prestonvanloon]

Excellent find! Thanks!