feat: add custom chains supported on Agent Kit

[CocDap]

The following configuration initializes a PolkadotAgentKit instance that exclusively connects to the Westend testnet. The agent is restricted to interact only with the westend chain:

const agent = new PolkadotAgentKit({
  privateKey: process.env.PRIVATE_KEY,
  chains: ['westend'], // Restrict agent access to the Westend chain only
});