Automatically define Block reward contract address in TokenBridge supply module

[vbaranov]

Motivation

Block reward contract address is hardcoded in TokenBridge supply module

Changelog

Extract Block reward contract address from blockRewardContract property of TokenBridge contract

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 a49649e4-e9e8-4694-955c-012e7665a469

• 0 of 10 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.06%) to 74.912%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/explorer/lib/explorer/chain/supply/token_bridge.ex	0	10	0.0%

Totals
Change from base Build 59bed80f-f472-496d-9262-32233ab43b5c:	-0.06%
Covered Lines:	5515
Relevant Lines:	7362

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💛 - Coveralls