Add cache support to eth_getBalance

[Nana-EC]

Problem

Currently the eth_getBalance queries the Mirror Node for the latest block.
Under high load a mirror node may limit RPS

Solution

To reduce the number of calls needed and therefore the load on a mirror node a cache may be added to eth_getBalance endpoint.

Mirror Node processes a block every 2 secs, however balance is updated in realtime based off of crypto transfers so a cache of 500ms may be a fair balance between optimization and data freshness.
This should be added as a configureable property

Alternatives

No response