Merge bitcoin/bitcoin#31583: rpc: add target to getmininginfo field and show next block info

a4df12323c doc: add release notes (Sjors Provoost)
c75872ffdd test: use DIFF_1_N_BITS in tool_signet_miner (tdb3)
4131f322ac test: check difficulty adjustment using alternate mainnet (Sjors Provoost)
c4f68c12e2 Use OP_0 for BIP34 padding in signet and tests (Sjors Provoost)
cf0a62878b rpc: add next to getmininginfo (Sjors Provoost)
2d18a078a2 rpc: add target and bits to getchainstates (Sjors Provoost)
f153f57acc rpc: add target and bits to getblockchaininfo (Sjors Provoost)
baa504fdfa rpc: add target to getmininginfo result (Sjors Provoost)
2a7bfebd5e Add target to getblock(header) in RPC and REST (Sjors Provoost)
341f932516 rpc: add GetTarget helper (Sjors Provoost)
d20d96fa41 test: use REGTEST_N_BITS in feature_block (tdb3)
7ddbed4f9f rpc: add nBits to getmininginfo (Sjors Provoost)
ba7b9f3d7b build: move pow and chain to bitcoin_common (Sjors Provoost)
c4cc9e3e9d consensus: add DeriveTarget() to pow.h (Sjors Provoost)

Pull request description:

  **tl&dr for consensus-code only reviewers**: the first commit splits `CheckProofOfWorkImpl()` in order to create a `DeriveTarget()` helper. The rest of this PR does not touch consensus code.

  There are three ways to represent the proof-of-work in a block:

  1. nBits
  2. Difficulty
  3. Target

  The latter notation is useful when you want to compare share work against either the pool target (to get paid) or network difficulty (found an actual block). E.g. for difficulty 1 which corresponds to an nBits value of `0x00ffff`:

  ```
  share hash: f6b973257df982284715b0c7a20640dad709d22b0b1a58f2f88d35886ea5ac45
  target:     7fffff0000000000000000000000000000000000000000000000000000000000
  ```

  It's immediately clear that the share is invalid because the hash is above the target.

  This type of logging is mostly done by the pool software. It's a nice extra convenience, but not very important. It impacts the following RPC calls:

  1. `getmininginfo` displays the `target` for the tip block
  2. `getblock` and `getblockheader` display the `target` for a specific block (ditto for their REST equivalents)

  The `getdifficulty` method is a bit useless in its current state, because what miners really want to know if the difficulty for the _next_ block. So I added a boolean argument `next` to `getdifficulty`. (These values are typically the same, except for the first block in a retarget period. On testnet3 / testnet4 they change when no block is found after 20 minutes).

  Similarly I added a `next` object to `getmininginfo` which shows `bit`, `difficulty` and `target` for the next block.

  In order to test the difficulty transition, an alternate mainnet chain with 2016 blocks was generated and used in `mining_mainnet.py`. The chain is deterministic except for its timestamp and nonce values, which are stored in `mainnet_alt.json`.

  As described at the top, this PR introduces a helper method `DeriveTarget()` which is split out from `CheckProofOfWorkImpl`. The proposed `checkblock` RPC in #31564 needs this helper method internally to figure out the consensus target.

  Finally, this PR moves `pow.cpp` and `chain.cpp` from `bitcoin_node` to `bitcoin_common`, in order to give `rpc/util.cpp` (which lives in `bitcoin_common`) access to `pow.h`.

ACKs for top commit:
  ismaelsadeeq:
    re-ACK a4df12323c
  tdb3:
    code review re ACK a4df12323c
  ryanofsky:
    Code review ACK a4df12323c. Only overall changes since last review were dropping new `gettarget` method and dropping changes to `getdifficulty`, but there were also various internal changes splitting and rearranging commits.

Tree-SHA512: edef5633590379c4be007ac96fd1deda8a5b9562ca6ff19fe377cb552b5166f3890d158554c249ab8345977a06da5df07866c9f42ac43ee83dfe3830c61cd169

doc/release-notes-31583.md

@@ -0,0 +1,9 @@
+Updated RPCs
+---
+- `getmininginfo` now returns `nBits` and the current target in the `target` field. It also returns a `next` object which specifies the `height`, `nBits`, `difficulty`, and `target` for the next block.
+- `getblock` and `getblockheader` now return the current target in the `target` field
+- `getblockchaininfo` and `getchainstates` now return `nBits` and the current target in the `target` field
+
+REST interface
+---
+- `GET /rest/block/<BLOCK-HASH>.json` and `GET /rest/headers/<BLOCK-HASH>.json` now return the current target in the `target` field