Create migrating-to-events-2.0.md (microsoft#3098)

* Create migrating-to-events-2.0.md

* Update migrating-to-events-2.0.md

Author: tevoinea

docs/migrating-to-events-2.0.md

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+In an effort to increase the reliability of our event and webhook delivery, we are reducing the amount of data being sent. To account for the missing data, we added a new endpoint for downloading an event's complete payload.
+
+## Example
+
+### Current Event format
+
+Here is an example webhook in Onefuzz version 8.1
+
+```jsonc
+{
+    "event": {
+      "container": "container-name",
+      "filename": "example.json",
+      "report": {
+          "asan_log": "example asan log",
+          "call_stack": [
+              "#0 line",
+              "#1 line",
+              "#2 line"
+          ],
+          "call_stack_sha256": "0000000000000000000000000000000000000000000000000000000000000000",
+          "crash_site": "example crash site",
+          "crash_type": "example crash report type",
+          "executable": "fuzz.exe",
+          "input_blob": {
+              "account": "contoso-storage-account",
+              "container": "crashes",
+              "name": "input.txt"
+          },
+          "input_sha256": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855",
+          "job_id": "00000000-0000-0000-0000-000000000000",
+          "onefuzz_version": "1.2.3",
+          "scariness_description": "example-scariness",
+          "scariness_score": 10,
+          "task_id": "00000000-0000-0000-0000-000000000000",
+          "tool_name": "libfuzzer",
+          "tool_version": "1.2.3"
+      }
+    },
+    "event_id": "00000000-0000-0000-0000-000000000000",
+    "event_type": "crash_reported",
+    "instance_id": "00000000-0000-0000-0000-000000000000",
+    "instance_name": "example",
+    "webhook_id": "00000000-0000-0000-0000-000000000000",
+    "sas_url": "https://example.com", // <------ THIS IS NEW
+    "version": "1.0"                  // <------ THIS IS NEW
+}
+```
+
+Notice the `sas_url` and `version` properties are new.
+
+### Events 2.0 sent event format
+
+When events 2.0 is released, the above event will be _sent_ like this:
+
+```json
+{
+    "event": {
+      "container": "container-name",
+      "report": {
+          "input_blob": {
+              "account": "contoso-storage-account",
+              "container": "crashes",
+              "name": "input.txt"
+          },
+          "job_id": "00000000-0000-0000-0000-000000000000",
+          "task_id": "00000000-0000-0000-0000-000000000000",
+      }
+    },
+    "event_id": "00000000-0000-0000-0000-000000000000",
+    "event_type": "crash_reported",
+    "instance_id": "00000000-0000-0000-0000-000000000000",
+    "instance_name": "example",
+    "webhook_id": "00000000-0000-0000-0000-000000000000",
+    "sas_url": "https://example.com",
+    "version": "2.0",
+    "expiration_date": "01/01/2025"
+}
+```
+
+You'll notice many fields are omitted. 
+
+**`event_id`, `event_type`, `instance_id`, `instance_name`, `webhook_id`, `sas_url`, `version`, `expiration_date` WILL ALWAYS BE INCLUDED**
+
+### How to retrieve the full event payload
+
+There are 3 options for retrieving the full event payload as it would have been sent in events 1.0
+
+1. Using the `sas_url` that is included in the event
+2. Using the new events API at `GET https://{onefuzz instance}/api/events` with a request body `{ "event_id": "00000000-0000-0000-0000-000000000000" }`
+3. Using the onefuzz cli `onefuzz events get "00000000-0000-0000-0000-000000000000"`
+
+The event payload data will only be retained until the expiration date. We are currently planning to retain event data for 90 days but this may change in the future.
+
+_**You can migrate your event handling code today to use any of those 3 options so that you will be unaffected by the future breaking change.**_