API endpoint: sum of balances except burning address

[vbaranov]

Motivation

A new endpoint to get a sum of the balances except burning address. Returns just a value.

/api?module=stats&action=coinsupply

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 7e4edc74-3e66-4efc-a79a-3522f90ec200

• 0 of 18 (0.0%) changed or added relevant lines in 5 files are covered.
• 3 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 74.88%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/api/rpc/stats_view.ex	0	1	0.0%
apps/block_scout_web/lib/block_scout_web/views/api/rpc/rpc_view.ex	0	2	0.0%
apps/block_scout_web/lib/block_scout_web/controllers/api/rpc/stats_controller.ex	0	3	0.0%
apps/explorer/lib/explorer/chain.ex	0	3	0.0%
apps/explorer/lib/explorer/chain/cache/address_sum_minus_burnt.ex	0	9	0.0%

Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/views/api/rpc/stats_view.ex	1	80.0%
apps/block_scout_web/lib/block_scout_web/controllers/api/rpc/stats_controller.ex	2	70.59%

Totals
Change from base Build 29f6f9c7-2348-4152-a9f0-6e4908fc977c:	-0.2%
Covered Lines:	5300
Relevant Lines:	7078

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💛 - Coveralls