Provide a DataCatalogIO to read/write Tags

Users need a mechanism to read structured Data Catalog tags or write tags to specific entities in a pipeline.

Typical use cases

*Read:* 

Use [Data Catalog search](https://cloud.google.com/data-catalog/docs/how-to/search-reference) to identify specific entities and take some action like generate metrics or send notifications etc.
 
*Write:*
* Add tags to entities in Data Catalog in a branch to update the results of the Pipeline
* Modify or update tags based on some dynamic criterion

Imported from Jira [BEAM-11615](https://issues.apache.org/jira/browse/BEAM-11615). Original Jira may contain additional context.
Reported by: anantdamle.