Stabilize GCP aggregator

[jpraynaud]

Make sure GCP aggregator is continuously producing snapshots:

• Provide db storage to non temporary directory for Mithril nodes in docker-compose.yaml (/home/curry/testnet)
• Keep track of snapshots in Mithril Aggregator
• Add Single Signatures store to aggregator #282
• Optimization: Update runtime rate of Aggregator and Signers
• Bug: Fix curry write rights to /home/curry/testnet and /home/curry/ipc directories (launch cardano-node with non root user)
• Bug: Fix Signer nodes not signing on Aggregator restart / thread 'main' panicked at 'Initializer not registered. Cannot participate as a signer.', /home/runner/work/mithril/mithril/mithril-core/src/stm.rs:575:17. May be fixed by Add chain observer interface  #281
• Question: Activate Cardano node as SPO on GCP or use other already existing SPO PoolId? (We use existing PollId for now and we will implement our own SPO later)