Coin total supply from DB API endpoint

[vbaranov]

Motivation

Calculate the total supply of POA Core coin and use that endpoint for CMC and exchanges.

Changelog

• the new JSON RPC endpoint in stats module
• the data is retrieved from ETS, ETS is updated every hour by default or configured via ADDRESS_SUM_CACHE_PERIOD ENV var

Addition of ENV vars to the docs blockscout/docs#10

API will be available at /api?module=stats&action=ethsupply
The similar API but from CoinGecko moved to /api?module=stats&action=ethsupplyexchange

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build b031af9d-65e5-405a-ba61-602e5187f1dd

• 12 of 15 (80.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 75.355%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/explorer/lib/explorer/chain/cache/address_sum.ex	6	9	66.67%

Totals
Change from base Build 5cd75094-825f-4ac2-a66b-05a400dcc719:	0.04%
Covered Lines:	5259
Relevant Lines:	6979

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💛 - Coveralls