Add tags and docs to event stream tests

This adds a `tags` property to event stream protocol test cases so
that tests can be filtered just like http request / response tests.

This also adds documentation for the trait.

Author: JordonPhillips

.changes/next-release/documentation-1738393fb185966c762e36f182d37808defc9c08.json

@@ -0,0 +1,7 @@
+{
+  "type": "documentation",
+  "description": "Added documentation for eventStreamTests",
+  "pull_requests": [
+    "[#2973](https://github.com/smithy-lang/smithy/pull/2973)"
+  ]
+}

.changes/next-release/feature-3a88783f9eb6834b538fbe729103db431f8993a9.json

@@ -0,0 +1,7 @@
+{
+  "type": "feature",
+  "description": "Added a tags property to eventStreamTests to allow filtering individual cases in the same way that http protocol tests can be.",
+  "pull_requests": [
+    "[#2973](https://github.com/smithy-lang/smithy/pull/2973)"
+  ]
+}