Closes #4.

Adds a Maven build pipeline that downloads datafusion_common.proto and datafusion.proto from apache/datafusion at a pinned tag (53.1.0, matching native/Cargo.toml) and generates Java classes from them via the Xolstice protobuf-maven-plugin. Generated classes ship inside the existing datafusion-java jar — the org.apache.datafusion.protobuf.* namespace from datafusion.proto, plus datafusion_common.* from datafusion_common.proto.

Also adds a bootstrap CONTRIBUTING.md whose primary content is the DataFusion / protobuf schema version-sync recipe. Bumping DataFusion now requires updating three things together: native/Cargo.toml, pom.xml's <datafusion.version>, and the SHA-512 pins on the two downloads. The recipe documents the exact commands to do that.

End-to-end functionality — Java code constructing a plan via these classes and executing it through SessionContext — is tracked separately in #8.

What's in this PR

• pom.xml: two new properties (<datafusion.version>=53.1.0, <protobuf.version>=3.25.5), protobuf-java runtime dep, os-maven-plugin build extension, two download-maven-plugin executions with SHA-512 pins, and Xolstice protobuf-maven-plugin for codegen.
• src/test/java/org/apache/datafusion/proto/ProtoGenerationTest.java: smoke test instantiating a generated class.
• CONTRIBUTING.md: new file with the version-sync recipe.

Not in this PR

• Module split (kept single-module for now; revisit when a proto-only consumer exists).
• Vendored protos (downloaded on first build, cached on ~/.m2 after that).
• JVM-side plan construction APIs, schema-fidelity tests, round-trip tests — see Build and execute Java-constructed protobuf plans via SessionContext #8.