Tracking: Support indexer/caching service in Zebra

[arya2]

Motivation

The indexer needs to notify clients when there are changes to the mempool or best chain with affected transactions.

This is more efficient than polling to check for changes, especially from the indexer's perspective, and has better usability for clients.

Design

Add a method to ChainTipChange in zebra-state for getting changes to the nullifier set, and a tonic IndexerRpcServer server for streaming responses to clients with server-sent events.

Implementation Plan

Preperation:

• [ ] https://github.com/ZcashFoundation/zebra/issues/7927
• [x] #8618

Indexing:

• [ ] #8837
• [ ] #8838

RPC methods:

• [x] #8619
• [ ] #8620
• [x] #8621
• [ ] #8622

Index Server / Cacher:

• [x] https://github.com/ZcashFoundation/zebra/issues/8520
• [x] https://github.com/ZcashFoundation/zebra/issues/8576
• [x] https://github.com/ZcashFoundation/zebra/issues/8580
• [ ] #8623
• [ ] #8628

Optional:

• [ ] Update TrustedChainSync to use the new RPC method to be notified when there’s a chain tip change or a change to the mempool
• [ ] Update zebra-scan to use RPC methods instead of a synced read state

[conradoplg]

I think this might be missing some of the notifications we need, namely when transactions are available and blocks are validated. See https://github.com/zcash/librustzcash/issues/1395#issuecomment-2211134913