Add codecov to project

[mantricjavier]

If you want to report a security issue, please follow our security policy: https://github.com/MANTRA-Chain/mantra-dex/blob/main/docs/SECURITY.md

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Improvement description

Add codecov.io to the project.

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Additional context

Add any other context here, code snippets, json responses, screenshots and so on.

Code sample

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