Add OP_PUSHBYTES to all the OP_RETURN outputs

[nchashch]

This will make the transactions pass the standardness checks.

It is possible to only do that for non-coinbase outputs, because coinbase doesn't need to adhere to standardness rules.

Though I think it would be better to do that for all OP_RETURNS for consistency/to make it less confusing. The cost is just one byte per output.

[nchashch]

This is already implemented in bip300301_enforcer by LayerTwo-Labs/bip300301_enforcer#52 .

So we need to update the specification to match.