refactor(sequencer): generate SequencerBlock after transaction exec…

…ution in proposal phase (#1562)

## Summary
refactor the sequencer app to generate and store the resulting
`SequencerBlock` after transaction execution even in the proposal phase.

## Background
previously, we were only generating the `SequencerBlock` in
`finalize_block`, however with the upcoming builder APIs (#1519) we
require the (proposed) `SequencerBlock` to be available after execution
in the proposal phase.

## Changes
- create a `post_execute_transactions` method and move the
after-execution logic that generates the `SequencerBlock` from
`finalize_block` to there.
- call this method after transaction execution in `process_proposal`. 
- if txs were executed in `prepare_proposal`,
`post_execute_transactions` is still called in `process_proposal`, as
the block hash is not available in `prepare_proposal`.

## Testing
existing unit tests pass, app logic was not changed, just refactored

## Related Issues
 related to #1322