backend: remove file sink

[m-kus]

TL;DR

Simplified the bridge node architecture by removing the file sink component and streamlining the API response structure.

What changed?

• Removed the file_sink.rs module which was responsible for writing sparse roots to JSON files
• Renamed mmr_db_path to db_path in AppConfig to better reflect its purpose
• Modified the ApiResponseBody enum:
  • Added an Empty variant for responses that don't need to return data
  • Removed the AddBlock variant as it's no longer needed
• Updated the add_block method in AppClient to return Result<(), anyhow::Error> instead of Result<SparseRoots, anyhow::Error>
• Simplified the indexer implementation by removing the sparse roots sink logic
• Removed related CLI arguments for sparse roots output directory and shard size

[m-kus]

• Add chain state indexing and RPC endpoint #383
• Thread safe MMR #379
• Expose endpoint to fetch multiple block headers #377
• Single writer/multiple reader MMR store #376
• backend: remove file sink #375 👈 (View in Graphite)
• Fix typo #382 : 2 other dependent PRs (#378 , #381 )
• main

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