refactor, blockstorage: Replace blocksdir arg

Add a blocks_dir field to the BlockManager options. Move functions
relying on the global gArgs to get the blocks_dir into the BlockManager
class.

This should eventually allow users of the BlockManager to not rely on
the global Args and instead pass in their own options.

src/kernel/blockmanager_opts.h

@@ -5,6 +5,8 @@
 #ifndef BITCOIN_KERNEL_BLOCKMANAGER_OPTS_H
 #define BITCOIN_KERNEL_BLOCKMANAGER_OPTS_H
 
+#include <util/fs.h>
+
 #include <cstdint>
 
 class CChainParams;
@@ -19,6 +21,7 @@ struct BlockManagerOpts {
     const CChainParams& chainparams;
     uint64_t prune_target{0};
     bool fast_prune{false};
+    const fs::path blocks_dir;
 };
 
 } // namespace kernel