Customize chain ID and token denomination in GevulotClient

[aleasims]

This allows to set chain ID and token denomination for a client:

let client = GevulotClientBuilder::new()
    .chain_id("my_chain")
    .denom("my_token")
    .build()
    .await?;

Default values are:

const DEFAULT_CHAIN_ID: &str = "gevulot";
const DEFAULT_TOKEN_DENOM: &str = "ucredit";