isthmus: operator fee

[yuwen01]

Adds the operator fee, as described in this design doc and this spec

Here, I propose that these changes be included in the Holocene hardfork instead of isthmus, like the spec says. This can easily be changed if necessary.

[rakita]

The general policy is not to merge things that are not scheduled for inclusion, so let us wait for it.

[codspeed-hq]

CodSpeed Performance Report

Merging #1798 will not alter performance

_{Comparing yuwen01:operator-fee (2dc6635) with main (03ea24b)}

Summary

✅ 7 untouched benchmarks

[rakita]

Closing this as it is not planned, we can always open it when it gets included.