Add gas reporting job parallelization on CI

[mjlescano]

The objective of this PR is to be able to generate gas reports on the parallelized unit tests job. To achieve that we have to be able to generate multiple gas reports and merge them later on a separate job.

Right now, gas reporting is done by 2 steps:

1. Run npx hardhat test --gas, which will generate the file gasReporterOutput.json if running on CI
2. Then npx codechecks codechecks.unit.yml, which will generate a test-gas-used.log and upload it to Codechecks

And, for implementing parallelization, this PR generates different gasReporterOutput-N.json files, and then merges them into a single file to be able to upload it to codechecks using the npx hardhat test:merge-gas-reports gasReporterOutput-*.json command.

Pendings

• The merge of the gasReporterOutput.json files has different results compared when we generate it in a single run. This has to be fixed before merging this PR