add cache handlers for individual items (blocks, microblocks, principals)

[rafaelcr]

We currently have two cache handlers that we use to create ETags for most of our endpoints:

1. Chain tip: Latest block/microblock hash
2. Mempool: Hash digest of all non-pruned mempool tx_ids

While these work great for endpoints which depend on global chain or mempool status like /extended/v1/blocks or /extended/v1/tx/mempool, they can incorrectly return a cache miss for others that are concerned with individual items. Some examples:

• /extended/v1/blocks/:block_hash would be a cache miss when the chain advances, even though the individual block would not have changed at all
• /extended/v1/tx/:tx_id similar (but already being fixed in feat: add cache control to /extended/v1/tx/:tx_id #1229)
• /extended/v1/address/:principal/transactions would be a cache miss even though that principal did not receive or send any new transactions
• /extended/v1/address/:principal/balances same as above
• etc...

We should create an individual item cache handlers for each of these (and more) use cases so we can maximize our number of cached responses

[smcclellan]

@rafaelcr Can this be closed? The cached/uncached percentage has been improving the last several months, at ~75% now.