feat(examples): add example for soap (#2386)

examples/soap-with-custom-function/README.md

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+# SOAP Load Testing Example
+
+Artillery doesn't have an official SOAP engine, but it's still possible to test SOAP with it. While building [a dedicated engine](https://www.artillery.io/blog/extend-artillery-by-creating-your-own-engines) is one option, you can also use custom functions and leverage the existing HTTP engine. This example shows you how to do that.
+
+## What the example does
+
+This example calls the SOAP server by using a SOAP client (node-soap) in a custom function. That custom function gets called with each VU execution. We also emit [custom metrics](https://www.artillery.io/docs/reference/extension-apis#custom-metrics-api) from the function to track the number of requests and responses, as well as the time taken to make the SOAP request.
+
+```
+soap.addNumbers.requests: ...................................................... 8
+soap.addNumbers.response_time:
+  min: ......................................................................... 2
+  max: ......................................................................... 9
+  mean: ........................................................................ 4.9
+  median: ...................................................................... 2
+  p95: ......................................................................... 7.9
+  p99: ......................................................................... 7.9
+soap.addNumbers.responses: ..................................................... 8
+```
+
+Notes:
+- The `callSoapOperation` function has been abstracted to allow calling other operations, should you wish to extend this example.
+- The creation of the client is also cached to prevent creating it for every virtual user.
+
+## Running the SOAP server
+
+We provide a very simple SOAP server for this example, containing the `AddNumbersService` with a single `addNumbers` operation.
+
+First, install the server dependencies:
+
+```
+cd server && npm install
+```
+
+After installing the dependencies, start the SOAP server:
+
+```
+node app.js
+```
+
+This command will start a Socket.IO server listening at http://localhost:8000/.
+
+## Running Artillery test
+
+Once the SOAP server is up and running, execute the test script:
+
+```
+npx artillery run soap.yml
+```

examples/soap-with-custom-function/package.json

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+{
+  "name": "soap-with-custom-function",
+  "version": "1.0.0",
+  "description": "",
+  "scripts": {
+    "test": "npx artillery run soap.yml"
+  },
+  "author": "",
+  "license": "ISC",
+  "dependencies": {
+    "soap": "^1.0.0"
+  }
+}