Add medium & new eventSource to campaign/experienceevent

## What are the schemas that are affected by the issue

> **https://ns.adobe.com/experience/campaign/experienceevent**

## What are examples of products that are impacted by the issue

> **Adobe Campaign**

## Requested enhancements

1. An experience event in Adobe Campaign can come from multiple sources (a message, a web page or an offer) and some of those sources can be in multiple formats.

Basically a message sent to a recipient can contains multiple types of contents (such as a text and and a html version). In that case the device is supposed to display one of the content to the recipient depending either on recipient preferences or device capabilities.

Tracking the `medium` at the source of the event is used by Adobe Campaign to provide reports to Marketers about the effectiveness and usage of each type of content.

```json
"https://ns.adobe.com/experience/campaign/medium": {
"title": "Medium",
"type": "string",
"description": "The medium is a combination of the source of the event (message, web, ...) and its format if applicable.",
"meta:enum": {
"message_text": "The source of the event is message in a text format.",
"message_html": "The source of the event is message in a html format.",
"message_line": "The source of the event is a line message.",
"web": "The source of the event is web tracking tag in a web page.",
"offer": "The source of the event is an offer."
}
}
```

2. Add new eventSource: **pipelined-tracker**

```json
"https://ns.adobe.com/experience/campaign/eventSource": {
"title": "Event Source",
"type": "string",
"meta:enum": {
...
"pipelined-tracker": "This event was generated via Pipelined generated from tracker",
},
```

**tracker** is a replacement of **trackinglogd**.

Locate the schema for https://ns.adobe.com/experience/campaign/experienceevent and inspect the existing eventSource definition. Add the requested medium property and pipelined-tracker enum entry, then run the repository's schema validation or tests to confirm the updated schema remains valid.

Written by the indexing model from the issue text.