fn is_exist(env: soroban_sdk::Env, hash: soroban_sdk::String) -> bool
fn store_cert(env: soroban_sdk::Env, hash: soroban_sdk::String) -> Certificate
fn update_cert(
env: soroban_sdk::Env,
hash: soroban_sdk::String,
new_hash: soroban_sdk::String,
) -> Certificate
fn get_cert(env: soroban_sdk::Env, hash: soroban_sdk::String) -> Certificate