Add ability to search for online accounts

[urtho]

Upstream issue : algorand#1629

Problem

There is no way to quickly fetch all online accounts[^1].

Solution

• Add is-online boolean filter to account search endpoint
• Add is-reward-eligible boolean filter to account search endpoint
• Add expires-in-hours filter that returns only accounts that will expire no later than expires-in-hours
• Add appropriate optional partial index (for is-online only)

Response model should include:

• total online stake
• total eligible online stake
• avg block time for the last 100 rounds in milliseconds

Notes:

• is-reward-eligible assumes is-online
• expires-in-hours should calculate avg block time over last 100 blocks using a query to block_header
• return only accounts with non-expired keys (as online flag persists past key expiry)
• expires-in-hours might need a better name like expires-in-hours-or-less but shorter ;)

[urtho]

Let's wait a bit for a confirmation on how suspended accounts look like (is-online:false + valid keyreg ?) so that we can show paused accounts in the results as well.

[urtho]

Thinking we should publish new , not yet merged functionality under different prefix instead of /v2. This would allow us not to break apps that used bleeding-edge stuff after the feature got renamed during the final merge.