SDK: Add contract binding for get_user_groups (Savings contract)

[A6dulmalik]

Implementation path: sdk/src/savings/get-user-groups.ts (tests: sdk/src/savings/__tests__/get-user-groups.test.ts)

Add an SDK binding for get_user_groups on the Savings contract, which returns all group IDs a user belongs to.

• Maps return type (Vec<String> → string[])
• Includes unit test asserting list updates after joining groups
• Note: distinct from the Registry contract's get_user_groups which returns Vec<Address>
• Includes usage documentation

Reference: get_user_groups in Savings.