test: optimize and add coverage job to ./yml flow

[radomir-sebek]

Notable changes:

• added .github/workflows/coverage.yml that will not be bocking any PRs, rather trigger test coverage calculations on demand or on push/merge to main branch
• inside hardhat.node.config.ts, there was a strange logic for setting parameter of solidity compiler called viaIR. Namely, if this value is false, then upon running npm run coverage will throw compile error

Compilation:
============

UnimplementedFeatureError: Copying of type struct ProfileLib.OperatorFee memory[] memory to storage not yet supported.


Error in plugin solidity-coverage: HardhatError: HH600: Compilation failed

that seems to goes deep into argotorg/solidity#3446, which is beyond my knowledge.

After noticing that in compiler_config.json this parameter is set to true, I decided to update this flag in scope of coverage reporting as well.
Basically, I couldn't find better way to get coverage to work (tried updating solidity-coverage hw did not helped), and settingviaIR to true seemed like legit option.

• SOLIDITY_COVERAGE=true
  This was recommended based on here

• final html report should look like this:

[zsculac]

Thanks @radomir-sebek, nice work. Merging this PR now