feat(integration): Add billing_entity_id to integration collection mapping

[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 the first PR where we add a billing_entity_id field to the integration_collection_mappings along with the proper indexes and validations.

There will be two follow-up PR:

1. A PR which update the integration_mappings similarly.
2. A PR which update the integration payload services to define a lookup and fallback strategy based on the available mappings.

[groyoh]

Thanks for the review @julienbourdeau 🙏