added methods (privateKeyToAddress, parseAndValidatePrivateKey,and privateKeyToPublicKey) to web3.eth.accounts

[SantiagoDevRel]

Description

Issue 6620

Please include a summary of the changes and be sure to follow our Contribution Guidelines.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run lint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:coverage and my test cases cover all the lines and branches of the added code.
• I ran npm run build and tested dist/web3.min.js in a browser.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.
• I have updated the CHANGELOG.md file in the root folder.
• I have linked Issue(s) with this PR in "Linked Issues" menu.

[codecov]

Codecov Report

Merging #6621 (26bc017) into 4.x (20cc5da) will not change coverage.
The diff coverage is n/a.

❗ Current head 26bc017 differs from pull request most recent head 879a012. Consider uploading reports for the commit 879a012 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##              4.x    #6621   +/-   ##
=======================================
  Coverage   89.36%   89.36%           
=======================================
  Files         215      215           
  Lines        8309     8309           
  Branches     2248     2248           
=======================================
  Hits         7425     7425           
  Misses        884      884           

Flag	Coverage Δ
UnitTests	89.36% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
web3	∅ <ø> (∅)
web3-core	∅ <ø> (∅)
web3-errors	∅ <ø> (∅)
web3-eth	∅ <ø> (∅)
web3-eth-abi	∅ <ø> (∅)
web3-eth-accounts	∅ <ø> (∅)
web3-eth-contract	∅ <ø> (∅)
web3-eth-ens	∅ <ø> (∅)
web3-eth-iban	∅ <ø> (∅)
web3-eth-personal	∅ <ø> (∅)
web3-net	∅ <ø> (∅)
web3-providers-http	∅ <ø> (∅)
web3-providers-ipc	∅ <ø> (∅)
web3-providers-ws	∅ <ø> (∅)
web3-rpc-methods	∅ <ø> (∅)
web3-utils	∅ <ø> (∅)
web3-validator	∅ <ø> (∅)