lightningd: add watchman RPC handlers and typed watch API

Register watch_found, watch_revert, block_processed, revert_block_processed,
getwatchmanheight, and chaininfo handlers.  Add typed watchman_watch_* helpers
and the dispatch-table skeleton for future subsystem handlers.

Changelog-Added: lightningd: watchman module registers getwatchmanheight, chaininfo, watch_found, watch_revert, block_processed and revert_block_processed RPCs for bwatch plugin integration.

Author: sangbida