feat: add farming pool wasm upgrade path

[jamilahmadzai]

Closes #23

Bounty: GrantFox OSS campaign for issue #23

Summary

• add an admin-only upgrade(new_wasm_hash) entrypoint to farming-pool that emits the new hash and hot-swaps the current contract WASM
• add SCHEMA_VERSION, schema_version, and an admin-gated migrate placeholder for future storage migrations
• add factory upgrade_pool(pool_id, new_wasm_hash) for pool-by-pool upgrades without changing the factory default pool WASM hash
• cover direct upgrade auth/state preservation and factory-mediated upgrade behavior in tests

Validation

• cargo fmt --all -- --check
• cargo build
• cargo test -p farming-pool upgrade -- --nocapture
• cargo test -p factory upgrade_pool -- --nocapture
• cargo test -p farming-pool -- --nocapture
• cargo test -p factory -- --nocapture
• cargo test --workspace -- --nocapture
• git diff --check

Notes

• existing Soroban Events::publish deprecation warnings remain unchanged
• factory upgrades are intentionally pool-specific and do not mutate stored WasmHash