build: add examples module on a multi-module Maven build

[andygrove]

Which issue does this PR close?

• Closes #.

Rationale for this change

The repository lacks runnable end-to-end examples. Code snippets in the docs are easy to drift out of sync with the API: there is no build step that fails when a public method is renamed or removed.

Adding an examples/ Maven module that depends on the library lets the reactor compile every example on each build, so they cannot fall behind the API. Doing this requires the repo to be a multi-module Maven project; while we're there, the parent POM gets shared dependencyManagement and plugin versions so child modules stay terse.

What changes are included in this PR?

Multi-module restructure:

• Root pom.xml becomes the parent (datafusion-java-parent, packaging=pom) with dependencyManagement for arrow, protobuf, junit, and the library itself.
• core/ is a new directory holding the existing library (datafusion-java). src/ moves to core/src/.
• examples/ is a new module (datafusion-java-examples) depending on the library via ${project.version}. It wires exec-maven-plugin so each example launches with the right java.library.path and --add-opens flags.
• native/, proto/, Makefile, and mvnw stay at the repo root unchanged.
• Surefire's java.library.path now uses ${maven.multiModuleProjectDirectory} so it resolves under the reactor regardless of which module Maven is invoked from.
• apache-rat-plugin runs only at the root (<inherited>false</inherited>); the rat exclude list is unchanged.

Three runnable examples under examples/src/main/java/org/apache/datafusion/examples/:

• SqlQueryExample — registerCsv + a SQL GROUP BY aggregation.
• DataFrameExample — readCsv → filter / select / withColumnRenamed / distinct → writeParquet(singleFileOutput) → readParquet round-trip.
• ProtoPlanExample — build a LogicalPlanNode directly via the generated protobuf classes and execute it through SessionContext.fromProto.

Each example creates its own throwaway data in a temp dir and cleans up, so no external fixtures (TPC-H, etc.) are required.

Docs: docs/source/contributor-guide/development.md is updated with the new repo layout and a "Running an example" section that documents the ./mvnw install -DskipTests + exec:exec flow.

Are these changes tested?

• The full JVM test suite (./mvnw test) still passes against the relocated core/src/test/ sources — 61 tests run, 0 failures (12 skipped, same skip pattern as main when TPC-H data is absent).
• Each example was executed end-to-end via ./mvnw -pl :datafusion-java-examples exec:exec and produces expected output:
  • SqlQueryExample prints HIGH 3 215 / MEDIUM 1 60 / LOW 1 25.
  • DataFrameExample prints a 3-row deduped table and Round-tripped row count: 3.
  • ProtoPlanExample prints 42 7.
• spotless:check and the reactor build are clean across all three modules.

Are there any user-facing changes?

• The published library artifact (org.apache.datafusion:datafusion-java) is unchanged — same groupId, artifactId, version, and package contents.
• The repo layout changes: source paths move from src/main/java/... to core/src/main/java/.... IDE projects pointing at the old location need to re-import.
• New datafusion-java-examples artifact exists but is marked maven.install.skip=true / maven.deploy.skip=true and is not intended for distribution.