Add custom_attributes field to OrganizationMembership #1293

Job	Run time
Test Python 3.10	25s
Test Python 3.11	23s
Test Python 3.9	21s
Test Python 3.14	42s
Test Python 3.12	26s
Test Python 3.13	21s
Test Python 3.8	27s
Smoke test (Python ${{ matrix.python }})	0s
	3m 5s

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