A message literal, pkg.Message{field: value, ?other: maybe}.
message is the message's type: its CEL.TypeProvider's name for it,
or a well-known type's full name, such as "google.protobuf.Int64Value".
Each field has its name and value node; optional is true for a field
written with ?, which is set only when its optional value is present.
Summary
Types
@type t() :: %CEL.Expr.MessageLiteral{ fields: [CEL.Expr.message_field()], id: CEL.Checked.expr_id(), message: CEL.TypeProvider.name(), type: CEL.Expr.type() }