Add Keizai to developer tools

docs/developer-tools.mdx

@@ -32,3 +32,7 @@ An IDE where you can write, deploy, and invoke contracts on the Stellar network
 ### [SubQuery](https://subquery.network/)
 
 An open-source and flexible data indexer that supports both Stellar and Soroban. SubQuery allows users to build a custom API for their dapp or smart contract in minutes and provides clean indexed data to any service. You can index ledgers, transactions, operations, and effects from Stellar, and transactions and events from Soroban, into your own Postgres database. Read the [quick start guide here](https://academy.subquery.network/quickstart/quickstart_chains/stellar.html).
+
+### [Keizai](https://keizai.dev/)
+
+Keizai is an open-source postman-inspired application, designed to simplify and elevate the testing process for developers working with Soroban smart contracts.