Add additional field "description" to counter's endpoint

[sevenzing]

Tooltip texts for all number charts:

1. Average block time
   Description: Average time taken in seconds for a block to be included in the blockchain

2. Total accounts
   Description: Number of EOAs that sent at least 1 transaction

3. Total tokens
   Description: Number of all token contracts

4. Completed txns
   Description: Number of transactions with success status

5. Total blocks
   Description: Number of blocks over all time

6. Total txns
   Description: All transactions including pending, dropped, replaced, failed transactions

7. Number of deployed contracts today
   Description: Number of deployed contracts today

8. Total contracts
   Description: Number of contracts

9. Total verified contracts
   Description: Number of verified contracts

10. Number of verified contracts today
    Description: Number of contracts verified today

11. Total native coin transfers
    Description: Number of transactions with the transfer of the native coin

12. Total addresses
    Description: Number of addresses that participated in the blockchain

[sevenzing]

Testing stand avaliable on

https://blockscout-main.k8s-dev.blockscout.com/stats
https://stats-test.k8s-dev.blockscout.com/api/v1/counters

[yvaskov]

Frontend task blockscout/frontend#1151 is ready for release and approved