Shell: mempool: indicate needed fees to be considered by full mempool

Context

When an operation is rejected with the Rejected_by_full_mempool error, the error now indicates the fee that the operation would need to be included in the full mempool. This feature used to be present, but was lost in !6787 (merged) due to the rework of mempool bounding.

Fixes the "Full mempool" item of #5197 (closed).

This MR depends on !8573 (merged).

Manually testing the MR

Checklist

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