Add reason and invited fields to SessionStartEvent (#51070)

In addition to adding the new fields and populating them, this
also updates tsh kube exec to include a --kube-namespace flag similar
to tsh kube login to make the command easier and more intuitive to use.

Closes #51019.

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

@@ -379,6 +379,12 @@ message SessionStart {
   reserved "AccessRequests";
   reserved 11;
   // reserved jsontag "access_requests"
+
+  // Invited is a list of invited users to this session.
+  repeated string Invited = 12 [(gogoproto.jsontag) = "invited,omitempty"];
+
+  // Reason is the reason for starting this session.
+  string Reason = 13 [(gogoproto.jsontag) = "reason,omitempty"];
 }
 
 // SessionJoin emitted when another user joins a session

api/types/events/events.go

@@ -295,6 +295,7 @@ func (m *SessionStart) TrimToMaxSize(maxSize int) AuditEvent {
 
 	out := utils.CloneProtoMsg(m)
 	out.InitialCommand = nil
+	out.Invited = nil
 
 	maxSize = adjustedMaxSize(out, maxSize)
 
@@ -303,6 +304,10 @@ func (m *SessionStart) TrimToMaxSize(maxSize int) AuditEvent {
 
 	out.InitialCommand = trimStrSlice(m.InitialCommand, maxFieldSize)
 
+	customFieldsCount = nonEmptyStrsInSlice(m.Invited)
+	maxFieldSize = maxSizePerField(maxSize, customFieldsCount)
+	out.Invited = trimStrSlice(m.Invited, maxFieldSize)
+
 	return out
 }