Function identifier_membership 

pub async fn identifier_membership(
    conn: &impl DbQuery,
    inbox_id: &str,
    identifier: &Identifier,
    scw_verifier: &impl SmartContractSignatureVerifier,
) -> Result<Option<bool>, IdentityUpdateError>

Whether the association state of the inbox in the database holds the identifier, an Ethereum address in any letter case, or None when the database has no identity update of the inbox. Computing the state caches it in the database.