Add custom_attributes field to OrganizationMembership #1294

Job	Run time
Test Python 3.14	47s
Test Python 3.10	24s
Test Python 3.11	23s
Test Python 3.12	28s
Test Python 3.8	29s
Test Python 3.9	22s
Test Python 3.13	23s
Smoke test (Python ${{ matrix.python }})	0s
	3m 16s

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