mining: add interrupt()

Both waitTipChanged() and createNewBlock() can take a long time to
return. Add a way for clients to interrupt them.

The new m_interrupt_mining is safely accessed with a lock on
m_tip_block_mutex, but it has no guard annotation. A more thorough
solution is discussed here:
https://github.com/bitcoin/bitcoin/pull/34184#discussion_r2743566474

src/interfaces/mining.h

@@ -116,7 +116,7 @@ public:
      * @param[in] timeout     how long to wait for a new tip (default is forever)
      *
      * @retval BlockRef hash and height of the current chain tip after this call.
-     * @retval std::nullopt if the node is shut down.
+     * @retval std::nullopt if the node is shut down or interrupt() is called.
      */
     virtual std::optional<BlockRef> waitTipChanged(uint256 current_tip, MillisecondsDouble timeout = MillisecondsDouble::max()) = 0;
 
@@ -130,10 +130,15 @@ public:
      *                     regtest and signets with only one miner, as these
      *                     could stall.
      * @retval BlockTemplate a block template.
-     * @retval std::nullptr if the node is shut down.
+     * @retval std::nullptr if the node is shut down or interrupt() is called.
      */
     virtual std::unique_ptr<BlockTemplate> createNewBlock(const node::BlockCreateOptions& options = {}, bool cooldown = true) = 0;
 
+    /**
+     * Interrupts createNewBlock and waitTipChanged.
+     */
+    virtual void interrupt() = 0;
+
     /**
      * Checks if a given block is valid.
      *