GCSFileSystem requires gcp extra at lookup time while S3FileSystem does not

There's an inconsistency in how `FileSystems.get_filesystem()` handles missing optional dependencies between GCS and S3.

### Current Behavior

**S3 (without `aws` extra):**
```python
>>> from apache_beam.io import filesystems
>>> filesystems.FileSystems.get_filesystem("s3://blah")

```
Returns the filesystem object; validation happens later when the filesystem is actually used.

**GCS (without `gcp` extra):**
```python
>>> from apache_beam.io import filesystems
>>> filesystems.FileSystems.get_filesystem("gcs://blah")
ValueError: Unable to get filesystem from specified path, please use the correct path or ensure the required dependency is installed, e.g., pip install apache-beam[gcp]. Path specified: gcs://blah
```
Raises immediately because `GCSFileSystem` isn't registered as a subclass.

### Proposed Behavior

Both should behave consistently. GCSFileSystem should be returned from `get_filesystem()` like S3FileSystem, allowing callers to validate dependencies when the filesystem is actually used rather than at lookup time.

### Why This Matters

- Inconsistent API behavior is confusing
- Code that handles multiple filesystem types can't catch/handle GCS gracefully
- Dependency validation at usage time (not lookup time) allows for better error handling and lazy loading patterns

### Environment

- Apache Beam version: 2.70.0
- Python version: 3.11

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*Generated by Claude Code, confirmed by @hjtran*