chore(demo): upgradable realm demo

[mconcat]

Added upgradable realm demo.

• admin: main entrypoint for users and controller contract for store and logic
• store: persistent storage layer across migration steps
• logic: switchable logic implementation

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

[dongwon8247]

It's our initial approach to tackle to the upgradable realm feature. Please have a look. @moul @zivkovicmilos @Kouteki

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.79%. Comparing base (0246761) to head (31dbc62).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3147      +/-   ##
==========================================
- Coverage   63.80%   63.79%   -0.02%     
==========================================
  Files         549      549              
  Lines       78833    78833              
==========================================
- Hits        50300    50291       -9     
- Misses      25144    25152       +8     
- Partials     3389     3390       +1     

Flag	Coverage Δ
contribs/gnodev	61.16% <ø> (ø)
contribs/gnofaucet	15.77% <ø> (ø)
gnovm	67.94% <ø> (+<0.01%)	⬆️
misc/genstd	79.72% <ø> (ø)
tm2	62.39% <ø> (-0.04%)	⬇️

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[moul]

We should consolidate all the upgrade demos in one location, and use distinctive names. If you'd like, I can move my other demos to a different folder. Ideally, we should also maintain a shared README that outlines the differences in each implementation; I believe I have already created one.