only get export Fields.

cmd/compile/internal/gc/dump.go.

NewErrUnsupportedType returns new ErrUnsupportedType.

req must be struct(Not pointer to struct), or return nil(tt.Field() will panic) key for tag , db or json, if key is empty, use field name instead.

ReflectValue converts interface{} to reflect.Value based on string type.

ErrUnsupportedType ...