build: move rpc/request.cpp from util to common

Summary: This aligns with other rpc/* utility modules, aligns with Core, and is needed to prevent a linker error from being introduced in D18224

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18228