Implement and test `getMultipleStates` method for Stub API.

[ArunRawat404]

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Issue

#453

Description

This PR implements the getMultipleStates method to ChaincodeStub and includes the following updates:

1. Implementation of getMultipleStates(...keys) method to retrieve multiple state values from the given keys.
2. Handler support for this via handleGetMultipleStates and handleOneSendGetMultipleStates in handler.js.
3. Unit tests for getMultipleStates to verify:
   • It returns correct results when keys are provided.
   • It returns an empty array when no keys are passed.
   • It throws on handler rejection.

This method mirrors functionality found in fabric-chaincode-go