Implement whitelist mode for farming pool contract

[Jonbaz32]

Closes #38

Description
This pull request implements the opt-in whitelist mode for the farming-pool contract, enabling pool admins to restrict stakes and lockups to pre-approved addresses (e.g., for invite-only campaigns or KYC compliance).

Key Features Implemented:

1. Consolidated PoolError definitions: Combined the duplicated PoolError definitions in types.rs into a single clean enum, adding NotWhitelisted = 15.
2. Whitelist DataKeys: Added WhitelistEnabled and Whitelisted(Address) variants to DataKey in types.rs.
3. Admin Actions: Added enable_whitelist, disable_whitelist, add_to_whitelist, remove_from_whitelist, and batch_add_to_whitelist (with a size check of max 50 addresses per call). All admin methods require admin authentication.
4. Guards & TTL Bumps:
   • Added guards in stake and lock_assets to reject non-whitelisted transactions when the whitelist mode is active.
   • Implemented persistent TTL bumping for Whitelisted(Address) records when queried or added.
5. Testing: Implemented comprehensive test coverage in test.rs validating default behavior (open pool), enabling/disabling, single user whitelisting/removal, batch whitelisting, and the batch limit boundary check.