chore(sequencer): refactor app (#1819)

## Summary
Refactored `<prepare/process>_proposal_execute_transactions` methods.

## Background
These methods previously contained a lot of shared code. This is meant
to eliminate this code duplication and also make the steps that are
happening in the proposals clearer and easier to understand.

## Changes
- Renamed `prepare_proposal_execute_transactions` to
`prepare_proposal_tx_execution` to better represent that this is the
execution step of the larger `prepare_proposal` method.
- Renamed `process_proposal_execute_transactions` to
`process_proposal_tx_execution`.
- Refactored block size checks and tx execution to shared helper
functions, which take a new enum `Proposal` containing information
necessary for either `Prepare` or `Process`.

## Testing
Passing all tests.

## Changelogs
No updates required

## Related Issues
closes #1785

crates/astria-sequencer/src/app/benchmarks.rs

@@ -93,7 +93,7 @@ impl Fixture {
 }
 
 #[divan::bench(max_time = MAX_TIME)]
-fn execute_transactions_prepare_proposal(bencher: divan::Bencher) {
+fn prepare_proposal_tx_execution(bencher: divan::Bencher) {
     let runtime = tokio::runtime::Builder::new_multi_thread()
         .enable_all()
         .build()
@@ -105,7 +105,7 @@ fn execute_transactions_prepare_proposal(bencher: divan::Bencher) {
             let (_tx_bytes, included_txs) = runtime.block_on(async {
                 fixture
                     .app
-                    .execute_transactions_prepare_proposal(constraints)
+                    .prepare_proposal_tx_execution(constraints)
                     .await
                     .unwrap()
             });