Added properties in DropdownButtonFormField to match DropdownButton (#36998)

* Added properties in DropdownFormField to match DropdownButton

* Minor style guide related changes
diff --git a/AUTHORS b/AUTHORS
index ccb6446..2a36d97 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -39,4 +39,5 @@
 Frederik Schweiger <mail@flschweiger.net>
 Martin Staadecker <machstg@gmail.com>
 Igor Katsuba <katsuba.igor@gmail.com>
-Diego Velásquez <diego.velasquez.lopez@gmail.com>
\ No newline at end of file
+Diego Velásquez <diego.velasquez.lopez@gmail.com>
+Sarbagya Dhaubanjar <mail@sarbagyastha.com.np>
\ No newline at end of file