Get all NFTs owned by account

[Szegoo]

Closes: #253

This PR adds two new functions to the RmrkApi.

• nftsOwnedBy
• propertiesOfNftsOwnedBy

The reason why I separated the two is that if we were to unite the two the return type would be too long and the readability of the code would suffer a bit. And also we already have this kind of separation by having two separate functions in the RmrkApi, nft_by_id and nft_properties.

Both of these functions have pagination implemented since the list of the NFTs a user owns may become very large.

The change I have made in the integration test:

• Since we already had a function inside the runtime API that returned all of the NFTs of a user inside the specific collection I have done some renaming in the integration tests to not cause confusion. The name of this function inside the integration test was getOwnedNfts and I renamed it to getOwnedNftsInCollection to emphasize that it doesn't return all of the NFTs. The function that I have added to the runtime API does actually get all of the NFTs so it has the name getOwnedNfts.

[Szegoo]

@ilionic I have added integration tests.

[ilionic]

Hey @Szegoo getting two failed integration tests:

  109 passing (7m)
  2 failing

  1) integration test: get owned NFTs
       "before all" hook for "fetch all NFTs owned by a user over multiple collections":
     Error: uniques.InUse

  2) integration test: get properties of owned NFTs
       "before all" hook for "fetch all the properites of the NFTs owned by a user over multiple collections":
     Error: uniques.InUse

[Szegoo]

Hey @Szegoo getting two failed integration tests:

Yes, this was a silly mistake, I used the same collection ids in the integration tests. Also, there seem to be some other fixes I need to do in the code.

[Szegoo]

@ilionic The integration tests should be passing now :)

[Szegoo]

@HashWarlock could you please review this?