fix(customers): accept and send id (external_id) in create/createBatch [BIL-1509]

[fjalil7]

Summary

• CreateCustomerParams.Builder now has an .id(String id) setter
• create and createBatch in CustomersResource now include id in the request body
• Wire-level tests added (CustomerCreateIdTest.java) using OkHttp MockWebServer

Test plan

• JAVA_HOME=/opt/homebrew/opt/openjdk@21 ./gradlew test — 30 tests pass
• Wire-level test captures the HTTP request body via MockWebServer and asserts id is present in create and createBatch

Closes BIL-1509

[linear-code]

BIL-1509