Internal c++ interfaces

The following interfaces are defined here:

• `Chain` — used by wallet to access blockchain and mempool state. Added in #14437, #14711, #15288, and #10973.

• `ChainClient` — used by node to start & stop Chain clients. Added in #14437.

• `Node` — used by GUI to start & stop bitcoin node. Added in #10244.

• `Wallet` — used by GUI to access wallets. Added in #10244.

• `Handler` — returned by handleEvent methods on interfaces above and used to manage lifetimes of event handlers.

• `Init` — used by multiprocess code to access interfaces above on startup. Added in #19160.

• `Ipc` — used by multiprocess code to access Init interface across processes. Added in #19160.

• `Rpc` — used by bitcoin-cli to be able to call RPC methods over a unix socket instead of TCP.

The interfaces above define boundaries between major components of bitcoin code (node, wallet, and gui), making it possible for them to run in different processes, and be tested, developed, and understood independently. These interfaces are not currently designed to be stable or to be used externally.