Expose collection attributes from `Inspect` trait (#1914)

# Description

- What does this PR do?

While working with `pallet_nfts` through `nonfungibles_v2` traits
`Inspect, Mutate`, I found out that once you have set the collection
attribute with `<Nfts as Mutate>::set_collection_attribute()`, it's not
possible to read it with `<Nfts as Inspect>::collection_attribute()`
since they use different `namespace` values. When setting the attribute,
`AttributeNamespace::Pallet` is used, while
`AttributeNamespace::CollectionOwner` is used when reading.

more context:
https://github.com/freeverseio/laos/issues/7#issuecomment-1766137370

This PR makes `item` an optional parameter in
`Inspect::system_attribute()`, to be able to read collection attributes.

- Why are these changes needed?

To be able to read collection level attributes when reading attributes
of the collection. It will be possible to read collection attributes by
passing `None` for `item`

- How were these changes implemented and what do they affect?

`NftsApi` is also affected and `NftsApi::system_attribute()` now accepts
optional `item` parameter.

## Breaking change

Because of the change in the `NftsApi::system_attribute()` method's
`item` param, parachains who integrated the `NftsApi` need to update
their API code and frontend integrations accordingly. AssetHubs are
unaffected since the NftsApi wasn't released on those parachains yet.