Add tag/label to workspace and repository variables

Currently workspace and repository variables are kept in a flat space with no option to organize or filter it.

Add a tag/label to each variable to keep meta-information about the variable.
Tags can be limited in size. A filtering option in the UI based on tag would be nice.

Using tags/labels the user can group variables in arbitrary ways, a few examples can be:

• sensitivity levels
• variables that apply to the same service
• the status of the variable across its lifetime: future use/in use/deprecated.