Expand nonfungible, nonfungibles traits functionality

[iorveth]

Issue

Functionalities, provided by nonfungible(s) trait interfaces looks relatively poor in comparison to features implemented in uniques pallet. https://github.com/paritytech/substrate/blob/master/frame/uniques/src/lib.rs

I think, that set_metadata and related trait functions looks like a good companions here. https://github.com/paritytech/substrate/blob/master/frame/support/src/traits/tokens/nonfungibles.rs#L164

Also, i hope, it would be good to have trait functions for clearing attributes and metadata here inside nonfungible(s) Mutate traits.