[random] add Fill<ObjectType>() function (#9097)

This commit adds template `Fill<ObjectType>(ObjectType &aObject)`
functions for both `Crypto` and `NonCrypto` random number generation
modules. These functions fill a given object with random byes.

tests/unit/test_ndproxy_table.cpp

@@ -50,7 +50,7 @@ Ip6::InterfaceIdentifier generateRandomIid(uint16_t aIndex)
 {
     Ip6::InterfaceIdentifier iid;
 
-    Random::NonCrypto::FillBuffer(iid.mFields.m8, sizeof(iid));
+    Random::NonCrypto::Fill(iid);
     iid.mFields.m16[3] = aIndex;
 
     return iid;