add print_latest_block_transactions toy hint

[tcoratger]

Should close #10.

The goal of this PR is simply to print the hashes of the transactions inside the blocks received by the ExEx via a custom hint inside the Cairo VM. The approach used is as follows:

• We define a custom hint print_latest_block_transactions that fetches the latest block in our SQLite and goes through all the transactions inside to print their hash. This is done each time we receive a new block notification so that we go through the tip of the chain each time to print the transaction hashes for this tip.
• When starting the cairo run, we define a particular hint processor that takes this hint into account.
• We also leave for now the possibility of running with Fibonacci (start_with_fibonacci vs start_with_transaction_logging) so we can play with both options at the start.