Whitelisting (tax exemption) feature according to prop #11824

[StrathCole]

Summary of changes

Introduce zone-based tax exemption.
Zones are always tax-free internally.
Optionally they can be set to be tax-free for incoming and/or outgoing txs.
Cross-Zone transactions are always taxed, unless cross-zone exemption is also set for the zone

Report of required housekeeping

• Github issue OR spec proposal link
• Added Binance whitelist migration
• Wrote tests
• Updated API documentation (client/lcd/swagger-ui/swagger.yaml)
• Added a relevant changelog entry: clog add [section] [stanza] [message]

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(FOR ADMIN) Before merging

• Added appropriate labels to PR
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)
• Confirm added tests are consistent with the intended behavior of changes
• Ensure all tests pass

[StrathCole]

One thing to note. I removed the old "contract" handling of whitelisting as the implementation made no sense. For a contract to be whitelisted, it needed to be added to the "Binance" zone. Also it didn't whitelist the contract (from my understanding) as intended, it just made sure the address needed to be a contract.

With this zone-approach contracts should now be able to be whitelisted as originally intended (incoming/outgoing = true).

[StrathCole]

For clarity here is the scheme I got sent of how it should work:

The "Exchanges" here are the "zones" in the implementation. The arrows represented the minimum necessary options (optional tax-exempt for incoming), optional exempt for outgoing / cross-zone was added for flexibility.

[StrathCole]

I would close this in favor of #393 if you agree @fragwuerdig ?
the new one uses gov v1 with legacy fallback, so should be less work to convert later.

[fragwuerdig]

👌