Add eth_blockNumber API endpoint to eth_rpc section

[vbaranov]

Closes #2586

Motivation

Changelog

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 c57ce4c5-3977-42a4-9291-4146d5a7fb95

• 0 of 8 (0.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.007%) to 75.62%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/api/rpc/block_view.ex	0	1	0.0%
apps/explorer/lib/explorer/eth_rpc.ex	0	7	0.0%

Files with Coverage Reduction	New Missed Lines	%
apps/explorer/lib/explorer/eth_rpc.ex	1	0.0%

Totals
Change from base Build 88d3a922-5da0-4575-ad14-095b7bda0b8b:	0.007%
Covered Lines:	5248
Relevant Lines:	6940

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💛 - Coveralls