Add CLI command to submit new Gov proposals, using Msgs

[amaury1093]

Summary

Add tx gov submit-proposal CLI command that uses sdk.Msgs.

Problem Definition

The current submit-proposal takes a content JSON, or flags that represent a content. We should migrate the CLI to the new submit proposal design.

Proposal

• Rename the old command to tx gov submit-legacy-proposal
• think about the UX of how to submit proposal using nested msgs.
  • The easiest way might be a JSON file that represents an array of Msgs, in proto-json

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