Replace byte arrays with hex encoding for json

[alan-ssvlabs]

closes #278

This PR creates custom marshallers and unmarshallers to change the encoding of bytes arrays in test json files to hex encoded strings

[GalRogozinski]

Good job!

I think it looks better now and has less confusing boilerplate code

Some comments I couldn't put in place due to VSCode bugs:

1. hash changes in beacon types encoding and messages encoding (under investigation)
2. In json_testutils consider the DRY (don't repeat yourself) approach that helps with future maintainability.

You could add the following helper functions, and then later only edit them as neccessary:

// hexStringFromJSON trims surrounding quotes from a JSON string value.
func hexStringFromJSON(data []byte) string {
	return strings.TrimSuffix(strings.TrimPrefix(string(data), "\""), "\"")
}

// tryUnmarshalByteArray tries to unmarshal a JSON array of bytes of a given length.
func tryUnmarshalByteArray(data []byte, expectedLen int) ([]byte, error) {
	var arr []byte
	if err := json.Unmarshal(data, &arr); err == nil {
		if len(arr) != expectedLen {
			return nil, fmt.Errorf("invalid array length: expected %d, got %d", expectedLen, len(arr))
		}
		return arr, nil
	}
	return nil, fmt.Errorf("not a valid byte array")
}

3. I spotted DataRound:0 changed to Round:1... we have leftovers form other PRs?
   Maybe it will get better after you fix conflicts

[MatheusFranco99]

Looks good to me! Good job!

[Zacholme7]

Lgtm thanks

[Zacholme7]

Follow up: this should also be applied to the entire repository, not just qbft

[GalRogozinski]

@Zacholme7
I see other folders changed
can you point out an example of a missing change

[dknopik]

Approving based on Zac's LGTM above

[y0sher]

lgtm