Support of Hyperledger Besu client

[vbaranov]

Motivation

There is no support of Hyperledger Besu client in Blockscout

Changelog

Add besu Ethereum JSON RPC variant:

export ETHEREUM_JSONRPC_VARIANT=besu

Related update in the docs blockscout/docs#22

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 e982e6ca-89bc-4e84-a226-4a73ba558601

• 0 of 65 (0.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.338%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/indexer/lib/indexer/fetcher/internal_transaction.ex	0	1	0.0%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc/besu/fetched_beneficiaries.ex	0	17	0.0%
apps/ethereum_jsonrpc/lib/ethereum_jsonrpc/besu.ex	0	47	0.0%

Totals
Change from base Build 67389e01-5271-4182-a20b-8a088566bbdf:	0%
Covered Lines:	5869
Relevant Lines:	7895

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💛 - Coveralls