feat: Add tags count to taxonomy serializer [FC-0036]

[ChrisChV]

Testing instructions

• You can test this change with https://github.com/openedx/frontend-app-course-authoring/pull/684#issue-1993737273
• Install this branch on the cms
• Follow the testing instructions in https://github.com/openedx/frontend-app-course-authoring/pull/684#issue-1993737273
• Verify that the tags count is showed in the delete dialog

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[bradenmacdonald]

@ChrisChV Please add a test that verifies this field is included in the REST API response(s)

[ChrisChV]

Please add a test that verifies this field is included in the REST API response(s)

@bradenmacdonald Sure. Added here https://github.com/openedx/openedx-learning/pull/125/commits/503bd0d1f6c5a06dee7feb79600017215e5fa6f4

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