SDK: Add contract binding for get_member (Savings contract)

[A6dulmalik]

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

Add an SDK binding for get_member, which retrieves a single member's data within a group.

• Maps Member struct including MemberStatus enum
• Maps error variant: NotMember
• Includes unit test asserting member data is correct after joining
• Includes usage documentation

Reference: get_member in Savings.