Revert "Revert to use BoundedVec for SyncCommitteePrepared"

[yrong]

This reverts commit 22274fa.
Only for diff compare and debug purpose for the issue here #811 (comment)

[yrong]

Another weird issue with const-generic-array is when running unit test without --release flag the memory access error found as following:

cargo test --verbose --package snowbridge-ethereum-beacon-client
Caused by:
  process didn't exit successfully: `/Users/yangrong/Projects/snowbridge/parachain/target/debug/deps/snowbridge_ethereum_beacon_client-0b7858a9df6f57aa` (signal: 10, SIGBUS: access to undefined memory)

and resolved with the --release option.

cargo test --release --verbose --package snowbridge-ethereum-beacon-client
test result: ok. 24 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.44s