[XNFT PoC] xtokens non-fungible multiassets support

[mrshiposha]

A change to the do_transfer_multiassets: support of the NonFungible fungibility for non-fee assets.

This PR doesn't change the API of extrinsics that use the configurable CurrencyId since it is a PoC. In the future, we could alter the amount argument to take the fungibility into account.

So only the following extrinsics can be used to transfer an NFT asset:

• transferMultiasset
• transferMultiassetWithFee
• transferMultiassets

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b45942e) 85.32% compared to head (dbcc789) 85.50%.

Files	Patch %	Lines
xtokens/src/lib.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #966      +/-   ##
==========================================
+ Coverage   85.32%   85.50%   +0.17%     
==========================================
  Files          88       88              
  Lines       10476    10533      +57     
==========================================
+ Hits         8939     9006      +67     
+ Misses       1537     1527      -10     

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[xlc]

need tests

[xlc]

some tests to ensure sending nft are actually working?