chore(bigquery-jdbc): dockerized proxy environment to run integration tests

[logachev]

This PR allows building regular & environment with proxy via Dockerfile.

Proxy environment restricts all internet traffic except maven.org, so it allows us to run entire IT suite with proxy to ensure it is operational.

I confirmed that #13539 passes majority of IT (failing tests are due to test setup rather than code, there will be separate PR to fix it & run it as a part of nightly workflow).

I also confirmed that if we had these tests enabled sooner, we'd catch #13494

[gemini-code-assist]

Code Review

This pull request introduces a Dockerized proxy environment for integration testing of the Java BigQuery JDBC driver. It adds multi-stage Docker builds, configures a Squid proxy with iptables rules to restrict raw network access (while allowing Maven Central), and updates the Maven configuration to support skipping the shade plugin. Feedback on these changes highlights two issues: first, the docker-proxy-session target in the Makefile references an undefined .docker-run-proxy target, which will cause execution to fail; second, the proxy readiness check in start-proxy.sh attempts to curl google.com, which is blocked by the iptables rules, and should instead use the allowed Maven Central domain.