Add a getPluginIdsByEntityType() method to src/Plugin/GroupContentEnablerManager

Problem/Motivation

The entitygroupfield → module (formerly "gcontent_field" from #2813405: Add a field to view and edit content's groups → ) needs to be able to find the right group content enabler plugin IDs for a given entity type + bundle.

Since this seems like something potentially useful to other Group-related modules, let's add it to Group core instead of doing our own thing in entitygroupfield.

Proposed resolution

Add a getPluginIdsByEntityType() method to the src/Plugin/GroupContentEnablerManagerInterface.php and the src/Plugin/GroupContentEnablerManager.php service ('plugin.manager.group_content_enabler').

Remaining tasks

3. RTBC.
4. Commit.

User interface changes

N/A

API changes

Adds a new getPluginIdsByEntityType() method to the src/Plugin/GroupContentEnablerManagerInterface.php and the src/Plugin/GroupContentEnablerManager.php service ('plugin.manager.group_content_enabler').

Data model changes

N/A

Release notes snippet

TBD, probably not.