feat(x/callback): Implementing Msg Server

[spoo-bar]

• Replacing repeated DecCoin with single Coin
• Replacing math.LegacyDec with sdk.Dec
• Modified CancelCallback proto to include contract address which was missing
• Implementing msg server methods
  • CancelCallback
  • RequestCallback
  • UpdateParams

[codecov[bot]]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (e12a8a1) 65.53% compared to head (10dd890) 65.59%.

Files	Patch %	Lines
x/callback/types/callback.go	0.00%	17 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feat/callback-module #513 +/- ## ======================================================== + Coverage 65.53% 65.59% +0.06% ======================================================== Files 67 67 Lines 3989 3985 -4 ======================================================== Hits 2614 2614 + Misses 1247 1243 -4 Partials 128 128 ```

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