Take sustainability fees from IBC shieldings

[sug0]

Related to #4790

Following #4939, we can repurpose the now useless overflow receiver middleware to take sustainability fees from IBC shieldings via payment addresses.

[brentstone]

@sug0 how will this work exactly? Can the fees be paid in any token or just the one from primary transaction? Will this be applied even at CLI level?

[sug0]

Can the fees be paid in any token or just the one from primary transaction?

Fees are deducted from ibc deposits in the deposited currency.

Will this be applied even at CLI level?

There is no CLI integration that would make sense here. This is implemented as an IBC middleware, that is triggered from an ICS-20 packet memo. What this means is that unless you voluntarily specify a memo in the counterparty chain's CLI (not even Namada's), no fees will be taken.

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Fees could also be taken through IBC deposits/withdraws from the MASP, and retained either in the PGF or Governance accounts. This would be a pretty minimal change, but it would require maintaining a table of shielding/unshielding gas costs per token.