Add Floci GCS filesystem tests

[electrum]

Description

Adds Floci-backed GCS filesystem tests, including coverage for encrypted operations and pre-signed URIs.

Release notes

(x) This is not user-visible or is docs only, and no release notes are required.

[Copilot]

Pull request overview

Adds Floci (floci-gcp) container-backed tests for the GCS filesystem implementation, including exercising pre-signed URIs and server-side encryption with customer-provided keys, and updates the GCS filesystem to generate pre-signed URLs that respect a configured custom endpoint (needed for emulators).

Changes:

• Introduce a FlociGcp Testcontainers wrapper and new TestGcsFileSystemFloci* test suites (plain + encryption).
• Add TestingSigningCredentials for signing pre-signed URLs in tests without relying on real GCP credentials.
• Plumb gcs.endpoint into GcsFileSystem and use it when generating V4 signed URLs; refactor GCS test initialization to accept custom config/auth; add needed test dependencies.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
testing/trino-testing-containers/src/main/java/io/trino/testing/containers/FlociGcp.java	Adds a Testcontainers wrapper for the floci-gcp emulator.
lib/trino-filesystem-gcs/src/test/java/io/trino/filesystem/gcs/TestingSigningCredentials.java	Adds test-only credentials implementing ServiceAccountSigner for signed URL generation.
lib/trino-filesystem-gcs/src/test/java/io/trino/filesystem/gcs/TestGcsFileSystemFlociWithEncryption.java	Adds encryption variant of the Floci-backed GCS filesystem test suite.
lib/trino-filesystem-gcs/src/test/java/io/trino/filesystem/gcs/TestGcsFileSystemFloci.java	Adds Floci-backed GCS filesystem tests using a container endpoint and signing creds.
lib/trino-filesystem-gcs/src/test/java/io/trino/filesystem/gcs/AbstractTestGcsFileSystem.java	Refactors initialization to allow passing a custom config and auth implementation.
lib/trino-filesystem-gcs/src/main/java/io/trino/filesystem/gcs/GcsFileSystemFactory.java	Passes configured endpoint into GcsFileSystem instances.
lib/trino-filesystem-gcs/src/main/java/io/trino/filesystem/gcs/GcsFileSystem.java	Uses configured endpoint hostname when generating pre-signed URLs.
lib/trino-filesystem-gcs/pom.xml	Adds Testcontainers JUnit Jupiter and testing-containers module as test dependencies.

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