feat(x/callback): Implement the storage of callback

[spoo-bar]

• Proto update to take in callback_height as int64 instead of uint64. As this is how the ctx.BlockHeight is stored
• Adding following keeper methods
  • GetAllCallbacks
  • GetCallbacksByHeight
  • ExistsCallback
  • DeleteCallback
  • SaveCallback

The store key where callbacks are stored: CallbackKey + CallbackHeight + ContractAddress + JobID

[codecov[bot]]

Codecov Report

Attention: 40 lines in your changes are missing coverage. Please review.

Comparison is base (7c47b4a) 66.17% compared to head (7db8e39) 65.51%.

:exclamation: Current head 7db8e39 differs from pull request most recent head 85dc2d4. Consider uploading reports for the commit 85dc2d4 to get more accurate results

Files	Patch %	Lines
x/callback/types/callback.go	0.00%	40 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feat/callback-module #500 +/- ## ======================================================== - Coverage 66.17% 65.51% -0.67% ======================================================== Files 66 67 +1 Lines 3941 3981 +40 ======================================================== Hits 2608 2608 - Misses 1206 1246 +40 Partials 127 127 ```

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