Let firrtl based applications run despite loading unknown annotations (#2387)

An application like barstools may contain a main that loads an annotations file containing
annotation classes that are not on it's classpath. This change allows unknown annotations
to be preserved by wrapping them in a UnrecognizedAnnotation. If annotations are then output
to a file, they will be unwrapped during serialization
This feature can be enabled via an AllowUnrecognizedAnnotations annotation

Co-authored-by: chick <chick.markley@sifive.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>