NFT spec

[fedekunze]

closes #4735

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry: clog add [section] [-t <tag>] [-m <msg>]

• Re-reviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4766 into billy/nft will increase coverage by <.01%.
The diff coverage is 100%.

@@              Coverage Diff              @@
##           billy/nft    #4766      +/-   ##
=============================================
+ Coverage      51.68%   51.69%   +<.01%     
=============================================
  Files            310      310              
  Lines          19661    19663       +2     
=============================================
+ Hits           10162    10164       +2     
  Misses          8782     8782              
  Partials         717      717

[okwme]

Changed base to billy/nft

[fedekunze]

@okwme hmm ideally this should be merged to master and independently from the code changes. The idea is for others to review it prior to merging the module itself