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.

Kokoro CIs show that the migrated tests are running.

b/500459077

[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.

[lqiu96]

not something we need to do in this PR, but I think we probably should convert this to proto3 at some time. Maybe we should create a ticket in the backlog

[lqiu96]

I think the changes do make sense. This breaks the dependence from BQStorage -> BQ.

Now IIUC the dependency graph is BQ depends on BQStorage (compile), BQStorage-It depends on BQStorage and BQ (test scope for both), BQStorage doesn't depend on anything.

Is this correct for the dependency graph?