Add Kintsugi tax invoice plugin v0.1.0

[JeffTax]

Initial contribution of the Kintsugi sales tax invoice plugin.

• Java OSGi invoice plugin (killbill-kintsugi) that delegates tax calculation to Kintsugi during invoice generation (POST /killbill/tax/estimate and /commit)
• HMAC-SHA256 request signing, re-invoice idempotency, and dual-path tax metadata (plugin properties / custom fields, optional Aviate billing-account lookup)
• TestNG fast suite with WireMock, healthcheck servlet, docker E2E toolkit, and CI via killbill/gh-actions-shared
• Maven coordinates: org.kill-bill.billing.plugin.java:kintsugi-plugin:0.1.0

See the README for install, tenant configuration, and troubleshooting. Maintainer notes for Central publish and docs.killbill.io live under docs/.

How to test

mvn clean verify

Requires JDK 11. Optional docker E2E: docker/run-e2e.sh (needs a Kintsugi test org).

[reshmabidikar]

I had to add the following dependency to the pom:

<dependency>
            <groupId>org.kill-bill.commons</groupId>
            <artifactId>killbill-utils</artifactId>
            <scope>runtime</scope>
</dependency>

Without this, the plugin results in the following exception:

123319-java.lang.ClassNotFoundException: org.killbill.commons.utils.collect.Iterables not found by org.kill-bill.billing.plugin.java.kintsugi-plugin [11]
123320- at org.killbill.billing.plugin.api.PluginProperties.findPluginPropertyValue(PluginProperties.java:89)
123321- at com.trykintsugi.killbill.internal.TaxInputMetadataLoader.resolveCustomerUsageType(TaxInputMetadataLoader.java:71)
123322- at com.trykintsugi.killbill.internal.TaxInputMetadataLoader.load(TaxInputMetadataLoader.java:55)
123323- at com.trykintsugi.killbill.internal.TaxMetadataResolver.resolve(TaxMetadataResolver.java:67)
123324- at com.trykintsugi.killbill.KintsugiInvoicePluginApi.getAdditionalInvoiceItems(KintsugiInvoicePluginApi.java:94)
123325- at com.trykintsugi.killbill.KintsugiInvoicePluginApi.getAdditionalInvoiceItems(KintsugiInvoicePluginApi.java:46)
123326- at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
123327- at java.base/java.lang.reflect.Method.invoke(Method.java:580)

[JeffTax]

Thanks @reshmabidikar — added org.kill-bill.commons:killbill-utils with runtime scope (same as stripe/vertex plugins) so PluginProperties can resolve Iterables inside the OSGi bundle.