feat(minor-interchain-token-service): register gateway token

[cjcobb23]

• add execute message to register gateway tokens

https://axelarnetwork.atlassian.net/browse/AXE-5693

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.19277% with 3 lines in your changes missing coverage. Please review.

Project coverage is 93.50%. Comparing base (a74a60a) to head (70fe584).

Files with missing lines	Patch %	Lines
...s/interchain-token-service/src/contract/execute.rs	97.43%	2 Missing :warning:
contracts/interchain-token-service/src/contract.rs	98.18%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #644 +/- ## ========================================== + Coverage 93.46% 93.50% +0.04% ========================================== Files 243 243 Lines 34228 34394 +166 ========================================== + Hits 31990 32159 +169 + Misses 2238 2235 -3 ```

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