Enhance the RefSelector component

This follows-up on the discovery issue: #290778 (closed)

In order to address the requirements of &4847 (closed), the RefSelector component needs to be changed in a few ways:

• Make it possible for the component to only fetch branches. To make this as flexible as possible, this could be done in the form of options to selectively enable/disable each type of entity that the component supports. By default, the component should keep fetching all the entities it supports.
• We'll need to be able to hide the section headers. Since tags and commits won't be displayed, we don't need to show the Branches header. A possible solution for this would build up on the previous task: when a single entity type is enabled, we could hide all headers and assume that proper context is given via higher-level strings.

Specs:

• Component fetches all entity types by default.
• Fetches selected entity types when provided.
• Hides section headers when a single entity type is provided.