Merge #18740: Remove g_rpc_node global

b3f7f375efb9a9ca9a7a4f2caf41fe3df2262520 refactor: Remove g_rpc_node global (Russell Yanofsky)
ccb5059ee89f6e8dc31ba5b82830b384890bb65e scripted-diff: Remove g_rpc_node references (Russell Yanofsky)
6fca33b2edc09ed62dab2323c780b31585de1750 refactor: Pass NodeContext to RPC and REST methods through util::Ref (Russell Yanofsky)
691c817b340d10e806dc3b1834d2a8fcc5e681fd Add util::Ref class as temporary alternative for c++17 std::any (Russell Yanofsky)

Pull request description:

  This PR removes the `g_rpc_node` global, to get same benefits we see removing other globals and make RPC code more testable, modular, and reusable.

  This uses a hybrid of the approaches suggested in #17548. Instead of using `std::any`, which isn't available in c++11, or `void*`, which isn't type safe, it uses a small new `util::Ref` helper class, which acts like a simplified `std::any` that only holds references, not values.

  Motivation for writing this was to provide an simpler alternative to #18647 by Harris Brakmić (brakmic) which avoids some shortcomings of that PR (https://github.com/bitcoin/bitcoin/pull/18647#issuecomment-617878826)

ACKs for top commit:
  MarcoFalke:
    re-ACK b3f7f375ef, only change is adding back const and more tests 🚾
  ajtowns:
    ACK b3f7f375efb9a9ca9a7a4f2caf41fe3df2262520

Tree-SHA512: 56292268a001bdbe34d641db1180c215351503966ff451e55cc96c9137f1d262225d7d7733de9c9da7ce7d7a4b34213a98c2476266b58c89dbbb0f3cb5aa5d70

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