Trait definitions for atomic write transactions

[adzialocha]

This PR introduces trait definitions for expressing atomic write transactions. This will be required to separate store methods into read and write queries and allow crash-resiliant write transactions across the whole p2panda stack.

Next Steps:

• Actually remove "write" methods from OperationStore and LogStore
• Implement new WriteToStore trait on Operation
• Decide where store trait definitons and implementations like OperationStore, LogStore, later probably EncryptionStore and AuthStore, etc. should live (either in p2panda-store or in their regarding crates)

📋 Checklist

• Add tests that cover your changes
• Add this PR to the Unreleased section in CHANGELOG.md
• New files contain a SPDX license header

[sandreae]

All looks very good, excited to start working with these new interfaces :-)