Improve handling of parsing OP_RETURN data

[KonradStaniec]

Currently op_return data for checkpoints is parsed here: https://github.com/babylonchain/babylon/blob/dev/x/btccheckpoint/types/btcutils.go#L207

This function should clearly document that it works only for standard OP_RETRUN transactions, and return error in non standard transactions.

Standard op_return transactions:

• must have only one op_return output
• must push only up to 80 bytes of datas