Add support for path with braces for Google Cloud Storage

At the moment we can not use braces in Google Cloud Storage paths, as explained [here](https://stackoverflow.com/questions/46977552/filebasedsource-not-able-to-understand-a-glob-corresponding-to-several-specific).

The path is backed by a file pattern defined as a Java glob and is then then expanded to a regex in _sdks/java/extensions/google-cloud-platform-core/src/main/java/org/apache/beam/sdk/util/GcsUtil.java_ in the _wildcardToRegexp_ function.

`gs://bucket/{file1,file2,file3}` should match `gs://bucket/file1`, `gs://bucket/file2` and `gs://bucket/file3`

Imported from Jira [BEAM-3224](https://issues.apache.org/jira/browse/BEAM-3224). Original Jira may contain additional context.
Reported by: benjzer.

Start in sdks/java/extensions/google-cloud-platform-core/src/main/java/org/apache/beam/sdk/util/GcsUtil.java, focusing on the wildcardToRegexp function and how it expands Java glob patterns. Verify the change against the stated gs://bucket/{file1,file2,file3} example; done means each listed path matches separately.

Written by the indexing model from the issue text.