refactor(bigquerystorage): move all integration tests to google-cloud-bigquerystorage-it

[jinseopkim0]

Moves all 22 integration test classes and helper/proto resources from google-cloud-bigquerystorage into a dedicated downstream integration test module (google-cloud-bigquerystorage-it).

Motivation

• Removes the test-scope dependency on google-cloud-bigquery from google-cloud-bigquerystorage.
• Prevents test code and .proto schema duplication between java-bigquery and java-bigquerystorage.
• Consolidates all end-to-end integration tests in a dedicated downstream module that depends on both google-cloud-bigquerystorage and google-cloud-bigquery.

[gemini-code-assist]

Code Review

This pull request introduces a new integration testing module, google-cloud-bigquerystorage-it, and moves the google-cloud-bigquery test dependency from the main storage module to this new integration test module. The feedback suggests managing the avro dependency version in the parent POM's <dependencyManagement> section rather than hardcoding it in the child module's pom.xml to ensure consistency.

[gemini-code-assist]

Avoid hardcoding dependency versions in child modules. To maintain consistency and simplify version upgrades, manage the version of avro in the parent POM's <dependencyManagement> section and omit the <version> tag here.

    <dependency>
      <groupId>org.apache.avro</groupId>
      <artifactId>avro</artifactId>
      <scope>test</scope>
    </dependency>