Pending txs per address API endpoint

[vbaranov]

Resolves #3144

Motivation

Missing useful in some applications, for instance for monitoring, API endpoint: pending transactions list per address

Changelog

Adding of pendingtxlist endpoint to Account module of API.. Example of request:
/api?module=account&action=pendingtxlist&address=0x8B99a5345819F00E4d6e90EB492858853314fcd1&offset=1000&page=1

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 24cb85bd-0f2e-4326-9daf-e2c96b2aff65

• 30 of 30 (100.0%) changed or added relevant lines in 3 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.08%) to 74.423%

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Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%

Totals
Change from base Build 02031898-6fc5-45f2-a08a-2e099dc79126:	0.08%
Covered Lines:	5898
Relevant Lines:	7925

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💛 - Coveralls