Implement getConfirmedBlock RPC method

[CriesofCarrots]

Problem

Solana users need a way to introspect particular blocks on the ledger via a request/response workflow. #6853 stubs out the initial methods. This ticket is to track remaining work.

Proposed Solution

Follow-up work:

• Plumb methods into solana-web3.js (fix: add getBlocksSince Connection method solana-web3.js#552 & fix: add getBlock Connection method solana-web3.js#555 & feat: Add getConfirmedBlock api solana-web3.js#562)
• Add persistent Transaction-status store to rocksdb (feat: rocksdb (persistent) transaction status cache #6958 )
• Add transaction fee to persistent Transaction-status store (feat: rocksdb (persistent) transaction status cache #6958 )
• Write transaction statuses to rocksdb after commit_transactions (both leader and other validator) (Write transaction status and fee into persistent store #7030 )
• Wrap transaction status write behind validator arg, to prevent rocksdb bloat on nodes that don't need/want this feature (Write transaction status and fee into persistent store #7030 )
• Parse get_slot_entries from blocktree to return real data from getConfirmedBlock (Add Blocktree api to get transactions by slot #6966 )
• Map Transaction-status to slot entries to flesh out getConfirmedBlock (Legitimately map transactions to statuses in blocktree #7011 )
• Return blockhash and previous blockhash in getConfirmedBlock (Add ConfirmedBlock struct, and rework Blocktree apis to include block… #7004 )
• Add parent slot to getConfirmedBlock (Add parent slot to getConfirmedBlock #7038 )
• Flesh out unit tests in rpc.rs vis-a-vis real data (Add getConfirmedBlock test to rpc #7120 )
• Update jsonrpc-api.md to document the new method (Add getConfirmedBlock to json-rpc docs #7046 )

[mvines]

Can we call this done?

[CriesofCarrots]

Can we call this done?

Almost. I'd still like a test in rpc.rs. I'll do that first thing and then close this.