feat: refactor storage into storage interface, add sqlite storage provider

[pschiel]

(WIP)

Proposal: use Storage Interface (re: sqlite topic)

• refactor Storage into using a StorageProvider.Interface
• move JSON logic into JsonStorageProvider
• add SqliteStorageProvider
• extend signature of list() with orderBy and limit (so providers can take advantage of index/sorting)
• configurable schema, allowing to extract certain (nested) keys from the data blob into separate columns
• sqlite command/util to export/import from/to JSON

Effect

• 100% backwards compatible - can switch storage via config
• open for more storage engines
• eliminate discussion about schema of preference

Migration steps

• opencode sqlite init to create new db
• opencode sqlite import to import json storage into sqlite
• set storage.backend to sqlite in opencode.json config