build: correctly declare api dependencies

[nielspardon]

Currently, both the isthmus and core projects are declaring all dependencies as implementation dependencies. Gradle has two types of dependencies api and implementation dependencies. From the Gradle documentation:

The api configuration should be used to declare dependencies which are exported by the library API, whereas the implementation configuration should be used to declare dependencies which are internal to the component.

https://docs.gradle.org/current/userguide/java_library_plugin.html#sec:java_library_separation

For the core project some of the protobuf Java classes are part of the Substrait Java API, e.g. com.google.protobuf.ByteString is exposed as part of the io.substrait.expression.Expression interface. Which means we should declare com.google.protobuf:protobuf-java as an api dependency instead of an implementation dependency.

Similarly, in the isthmus project classes coming from the dependency on the core project are part of the isthmus API and should be reported as an api dependency. Additionally, the Apache Calcite classes are part of the isthmus API and so the org.apache.calcite:calcite-core should also be declared as an api dependency.

In order to correctly declare these api dependencies we need to switch from the java to the java-library Gradle plugin.

If such dependencies are not declared as api dependencies Gradle projects depending on the core or isthmus libraries are forced to repeat the dependency declaration on the respective dependencies.

[CLAassistant]

All committers have signed the CLA.

[vbarua]

Thanks for this change, along with the super-clear explanation ✨