refactor: update EncodingConfig return type

Description

This PR updates the type returned from the EncodingConfig to be simappparams.EncodingConfig. This is to make sure that the MakeEncodingConfig method can be used whenever the simappparams.EncodingConfig type is used, such as in other tools like DJuno.

Author Checklist

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[ ] included the correct type prefix in the PR title
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[ ] followed the guidelines for building modules
[ ] included the necessary unit and integration tests
[ ] added a changelog entry to CHANGELOG.md
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[ ] updated the relevant documentation or specification
[ ] reviewed "Files changed" and left comments if necessary
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[ ] confirmed the correct type prefix in the PR title
[ ] confirmed ! in the type prefix if API or client breaking change
[ ] confirmed all author checklist items have been addressed
[ ] reviewed state machine logic
[ ] reviewed API design and naming
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[ ] reviewed tests and test coverage
[ ] manually tested (if applicable)

desmos-labs / desmos