SSMRun Audit Event: add invocation url (#41663)

This PR adds a new field in the SSMRun audit event: invocation url.

EC2 Auto Discover uses SSM to install teleport in the target instance.
An invocation is the execution of a Command in an Instance.
This URL points to that invocation and users can more easily debug what
went wrong and how they can fix in case of a failure.

api/proto/teleport/legacy/types/events/events.proto

@@ -5209,6 +5209,10 @@ message SSMRun {
   // StandardError contains the stderr of the executed command.
   // Only the first 24000 chars are returned.
   string StandardError = 9 [(gogoproto.jsontag) = "stderr"];
+
+  // InvocationURL is a link to AWS Web Console for this invocation.
+  // An invocation is the execution of a Command in an Instance.
+  string InvocationURL = 10 [(gogoproto.jsontag) = "invocation_url"];
 }
 
 // CassandraSession is emitted when a Cassandra client sends