Add Liquidity module

[dongsam]

Summary

Add a tendermint/liquidity module to the Gaia for Gravity DEX.

Proposal

The liquidity module serves AMM (Automated Market Makers) style decentralized liquidity providing and coin swap functions.
The module enables users to create a liquidity pool, make deposits and withdrawals, and request coin swaps from the liquidity pool.

The module used in https://gravitydex.io Incentivized testnet and all the functional parts has been completed.
Currently in the final stages of security auditing and we are improving the level of completion.
When the audit results come out, we will release the liquidity module version with it, and before that, we will open the draft PR first and rebase it when the final version is released.

• Liquidity Module Spec
• Liquidity Module Lite Paper (English)
• Proposal and milestone
• Swagger HTTP API doc
• godoc
• Client doc
• Performance Testing

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