annotations should support custom messages and classes

Update https://github.com/apache/incubator-beam/blob/python-sdk/sdks/python/apache_beam/utils/annotations.py to add 2 new features:

1. ability to customize message
2. ability to tag classes (not only functions)

Imported from Jira [BEAM-1081](https://issues.apache.org/jira/browse/BEAM-1081). Original Jira may contain additional context.
Reported by: altay.

Start with sdks/python/apache_beam/utils/annotations.py and review how annotations currently handle functions. Define the expected custom-message behavior and class-tagging behavior from the existing implementation and the linked Jira context. Done means both features work without regressing current function annotation behavior, with appropriate tests added for messages and classes.

Written by the indexing model from the issue text.