POST REST /customers endpoint not available in ACCS (SaaS)

## Context
- The API is available in PaaS REST API (Note: This PaaS endpoint is publicly accessible)
- The /customers POST REST endpoint is missing from ACCS (Note: All public-facing endpoints have been removed to allow for future rework if necessary)
- Several extensions published in the Exchange use the endpoint to synchronize Commerce users created in external systems (e.g., SAP, Hubspot, etc.).
## Issue
The missing endpoint in ACCS creates an incompatibility with ACCS for the starter kit (as well as for the published extensions in the Exchange built on top of it)
## Fix
- The /customers POST REST endpoint will be added to ACCS with restricted access (OAuth)
- ETA: The endpoint will be available in ACCS Sandbox in two weeks

No repository files or tests are named. Start by comparing the existing PaaS /customers POST endpoint with ACCS endpoint availability and access requirements. Done means ACCS exposes the POST /customers endpoint with restricted OAuth access and the starter kit and dependent Exchange extensions are compatible.

Written by the indexing model from the issue text.