feat(cli): add generator API for external CLI consumers

[rmorehig]

Summary

• add a new runGenerate command module that exposes generated Tinybird artifacts with a stable contract (type, name, relativePath, content)
• add tinybird generate CLI command with --json output and optional --output-dir file materialization
• export ./cli/commands/generate in package exports for programmatic consumption
• bump package version to 0.0.57

Testing

• pnpm vitest run src/cli/commands/generate.test.ts
• pnpm vitest run src/cli/commands/build.test.ts src/cli/commands/deploy.test.ts
• pnpm typecheck
• pnpm build