Add API to get balance of an address on a block_height for a ticker

[InfoIsland]

Summary

For indexer to effectively get balance on block for a particular ticker. It is good to have an batch API for it. It can largely reduced the rate for calling API, roughly by ~100x less. The shape of the API can looks like following:

request: {address: string, block_height: number, ticker: string}[]
response: {address: string, block_height: number, ticker: string, balance: string}[]

which request is an array of the queries, and response is an array of query results.

Reference

• The similar BIS api documentation is here
• The Indexer implementation for using such API is here