implement leo example command for lottery, tictactoe, token

[collinc97]

Create a new Leo example package in a new directory

Usage: leo example [OPTIONS] <COMMAND>

Commands:
  lottery    A public lottery program
  tictactoe  A standard tic-tac-toe program
  token      A transparent & shielded custom token program
  help       Print this message or the help of the given subcommand(s)

[codecov]

Codecov Report

Merging #2514 (e55f887) into testnet3 (af1f7f9) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           testnet3    #2514   +/-   ##
=========================================
  Coverage     79.02%   79.02%           
=========================================
  Files           157      157           
  Lines          4945     4945           
  Branches       4945     4945           
=========================================
  Hits           3908     3908           
  Misses         1037     1037           

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[d0cd]

LGTM!