code: Improve mempool of Starknet test app

[romac]

Our current mempool for the Starknet mock app is quite barebones and could use a few improvements, namely:

• Split out tx generation out of the Mempool actor and into its own actor (MempoolLoad or something)
• After a block is decided, only remove txs that were included in that block
• Allow configuring the MempoolLoad actor to generate load on the mempool in various ways
  • No load
  • Uniform load configured in TPS (transactions per second)
  • Non-uniform load to simulate peaks in tx ingestion, the frequency and amplitude of these peaks should be configurable
• Allow size of generated transactions to be configured (constant, uniformly distributed, other distributions)