Voluntary fees IBC middleware

[sug0]

Describe your changes

Closes #4944

Based on #4939
Review: https://github.com/namada-net/namada/pull/4968/files/ed70e61a6aa728c9efa2a42aca24068ebb4888b6..18eea0b7ed599f23df15713592869302a3722533

This PR reworks the now useless Osmosis Shielded Swaps IBC middleware, which was then used to transfer amounts overflowing the min output of a trade to a transparent account.

Its role is now to redirect some funds transferred to a payment address (with memoless IBC shieldings) to a transparent account, to collect IBC shielding fees.

In this PR, it is integrated with the existing Osmosis Shielded Swaps SDK functionality. In Namadillo, an IBC memo must be set when using Keplr to shield funds over to Namada. It should have the following structure:

{
  "namada": {
    "voluntary_fees": {
      "fee_receiver": "tnam1...",
      "new_received_amount": "123"
    }
  }
}

Where new_received_amount is the original amount minus the intended fee.

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

[mergify]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[grarco]

What if we want to pay a sus fee because the source is shielded? (So swapping from a shielded address to a transparent one)

[grarco]

Nevermind, build_ibc_transfer attaches the fee to the transfer data on the way out so we should be good

[grarco]

Shouldn't we overwrite the value of transfer.frontend_sus_fee to None when we build the memo (i.e. when we swap from a transparent to a shielded address)? Cause the build_ibc_transfer function seems to return an error in this case