[bitcoin/bitcoin] Introduce MempoolObserver interface to break «policy/fees -> txmempool -> policy/fees» circular dependency (#13949)

utACK 56ec018

One nit: I was wondering about the location of `mempool_observer.h`. If the file belongs in `src/interfaces`, where it currently resides, you may want to add an entry for the `MempoolObserver` interface to https://github.com/bitcoin/bitcoin/blob/master/src/interfaces/README.md, which contains a list of headers (not up to date).

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