Proxy contracts: methods decoding using implementation ABI

[vbaranov]

#1151 #2839

Motivation

Methods of implementation contract which called on proxy contract are not decoded

Changelog

If contract's ABI contains "implementation" method and it returns an address of existing on-chain contract Blokcscout will retrieve ABI of implementation from its DB (thus, implementation contract should anyways be verified in the Blockscout) and will try to decode methods using full ABI which comprises from proxy and implementations ABIs.

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 04aa3579-37be-48af-9dbb-367da204b66b

• 17 of 18 (94.44%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 74.955%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/explorer/lib/explorer/chain/transaction.ex	3	4	75.0%

Totals
Change from base Build 83d526b1-6a67-4965-847d-d1ae67d2e535:	0.08%
Covered Lines:	5869
Relevant Lines:	7830

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💛 - Coveralls