Read methods of implementation on proxy contract

[vbaranov]

#2839

Motivation

Missing useful read proxy contract feature.

Changelog

Note: in order to enable this feature both proxy and implementation contracts should be verified inn Blockscout and proxy contract should have "implementation" getter with the address of current implementation.

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 f3eaf00b-4cab-4670-b4a6-b974b34a9a95

• 40 of 43 (93.02%) changed or added relevant lines in 6 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.009%) to 74.347%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/address_view.ex	1	2	50.0%
apps/explorer/lib/explorer/chain.ex	16	17	94.12%
apps/explorer/lib/explorer/smart_contract/reader.ex	8	9	88.89%

Files with Coverage Reduction	New Missed Lines	%
apps/explorer/lib/explorer/chain.ex	2	75.45%
apps/block_scout_web/lib/block_scout_web/controllers/smart_contract_controller.ex	3	85.71%

Totals
Change from base Build 02031898-6fc5-45f2-a08a-2e099dc79126:	0.009%
Covered Lines:	5892
Relevant Lines:	7925

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💛 - Coveralls