feat(integration): Update billing entity mappings in integration payload

[groyoh]

Context

Currently, our customers cannot sync invoices to multiple accounting entities (subsidiaries) as our mapping is made at the organization level. However, in NetSuite, for instance, the tax and item mapping are made at the entity level. We need to allow our mapping to be defined at the entity level.

Description

This is a follow-up PR to #4445 and #4446.

This makes sure that each integration rely on the correct integration mapping or integration collection mapping following this order:

1. Mapping for item on billing entity
2. Fallback item on billing entity
3. Mapping for item on organization
4. Fallback item on organization

To make sure the behavior is correctly handled on all integrations, I'll heavily update the tests in an other PR.

[groyoh]

Thanks for the review @ivannovosad 🙏