SDK: Add contract binding and test for join_group (Savings contract)

[A6dulmalik]

Implementation path: sdk/src/savings/join-group.ts (tests: sdk/src/savings/__tests__/join-group.test.ts)

Add an SDK binding for join_group, which allows a user to join an open savings group.

• Maps input params: member, group_id
• Maps error variants: GroupNotFound, GroupNotAcceptingMembers, GroupIsFull, AlreadyMember
• Includes unit tests for success, duplicate join, full group, and non-open group
• Includes usage documentation

Reference: join_group in Savings.