fix: nft utils should be imported from common index

[andreabadesso]

Motivation

Should fix Incident #229

Error: Cannot find module '@wallet-service/common/src/utils/nft.utils'

Acceptance Criteria

• Importing directly from src works in dev and tests but fails in the compiled version, we should import from the exported index
• Added the processNftEvent to the mocked NftUtils in services tests

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged
• Make sure either the unit tests and/or the QA tests are capable of testing the new features
• Make sure you do not include new dependencies in the project unless strictly necessary and do not include dev-dependencies as production ones. More dependencies increase the possibility of one of them being hijacked and affecting us.