CHIA-1945 Accept a tx_filter param in get_block_header and exclude reward coins when the filter is not requested

[AmineKhaldi]

Purpose:

We tend to use get_block_header in paths where we don't care about the transactions filter, by passing empty additions and removals, yet when a transaction block contains reward coins, we end up computing a non-empty one. Now we accept a tx_filter param that allows us to skip the filter computation entirely.

Current Behavior:

We generate a non-empty transactions filter when we get called with a transaction block that has reward coins, even if we pass empty additions and removals, which we usually do to signal not caring about the filter.

New Behavior:

We generate an empty transactions filter when we get called with tx_filter as False, even if we get a transaction block that has reward coins (and whether was pass additions and removals or not).