docs: Added design doc for Util Service (#13958)

Signed-off-by: Derek Riley <derek.riley@swirldslabs.com>

hedera-node/docs/design/services/services.md

@@ -9,6 +9,7 @@ of what each of the different APIs are intended for.
   - [Schedule Service](schedule-service/schedule-service.md)
   - [Consensus Service](consensus-service/consensus-service.md)
   - [Network & Freeze Service](network-admin-service/network-admin-service.md)
+  - [Util Service](util-service/util-service.md)
 
 ## Service
 

hedera-node/docs/design/services/util-service/util-service.md

@@ -0,0 +1,69 @@
+# Util Service
+
+The Util Service is a service that handles utility operations. 
+Currently, the only functionality provided by this service is the generation of 
+pseudorandom numbers.
+
+### Table of Contents
+- [Util Service](#Util-Service)
+- [Protobuf Definitions](#Protobuf-Definitions)
+    - [UtilPrngTransactionBody](#UtilPrngTransactionBody)
+- [Handlers](#Handlers)
+- [Configuration](#Configuration)
+
+## Protobuf Definitions
+Protobuf, or Protocol Buffers, is a method of serializing structured data. 
+The Util Service uses it to define the structure of our transactions. Here are some of 
+the Protobuf definitions used in the Util Service:  
+
+- ```util_service.proto```: This file defines the UtilService which includes the prng RPC 
+for generating pseudorandom numbers.  
+- ```util_prng.proto```: This file defines the UtilPrngTransactionBody message which is
+used in the prng RPC. It includes a range field which, if provided and is positive, 
+returns a 32-bit pseudorandom number from the given range in the transaction record. 
+If not set or set to zero, will return a 384-bit pseudorandom number in the record.
+
+### UtilPrngTransactionBody
+The `UtilPrngTransactionBody` message is a crucial part of the `prng` RPC in the Util Service. 
+It is defined in the `util_prng.proto` file and is used to specify the range for the pseudorandom 
+number generation.
+
+```
+message UtilPrngTransactionBody {
+    int32 range = 1;
+}
+```
+
+The `UtilPrngTransactionBody` message contains a single field, `range`. This field is used to determine the range of the pseudorandom number that will be generated.
+
+- If `range` is provided and is positive, the Util Service will return a 32-bit pseudorandom number from the given range in the transaction record.
+- If `range` is not set or set to zero, the Util Service will return a 384-bit pseudorandom number in the transaction record.
+
+This message is used in the `prng` RPC of the `UtilService` defined in `util_service.proto`. 
+The `prng` RPC is responsible for generating pseudorandom numbers, and 
+the `UtilPrngTransactionBody` message provides the necessary input for this operation.
+
+## Handlers
+
+Handlers are responsible for executing the transactions. Each type of transaction has its 
+own handler. All the Handlers implement the TransactionHandler interface and provide 
+implementations of pureChecks, preHandle, handle, and calculateFees methods.
+
+- ```UtilHandlers.java```: This class includes a prngHandler which is an instance of 
+UtilPrngHandler.  
+
+- ```UtilPrngHandler.java```: This class is a TransactionHandler for handling UTIL_PRNG 
+transactions. It uses the n-3 running hash to generate a pseudo-random number. 
+The n-3 running hash is updated and maintained by the application, based on the 
+record files generated based on preceding transactions. In this way, the number is 
+both essentially unpredictable and deterministic.
+
+## Configuration
+```UtilPrngConfig``` is a configuration class used in the Hedera Hashgraph network. 
+This class is used to configure the behavior of the ```UtilPrngHandler```. The ```UtilPrngConfig``` 
+class is a record that contains a single boolean property, isEnabled. This property is 
+annotated with ```@ConfigProperty``` and ```@NetworkProperty```, indicating that it can be 
+configured via the network's configuration file. The isEnabled property determines whether 
+the ```UtilPrngHandler``` allows a transaction to proceed. If isEnabled is set to true, the 
+```UtilPrngHandler``` will allow the transaction. If isEnabled is set to false, 
+the ```UtilPrngHandler``` will not allow the transaction.