Introduces the mechanism for runtime-mutable plugin options: an option
marked dynamic can be changed via `lightning-cli setconfig <name> <val>`
without restarting clboss or lightningd. No option opts in yet -- this
is the foundation (the rebalancer mode selector is the first consumer).
- Boss::Msg::ManifestOption gains a bool dynamic field (default false,
preserving the existing startup-only contract).
- Boss::Mod::Manifester emits the per-option dynamic flag in the
getmanifest response, so lightningd knows to forward setconfig for
that option.
- New Boss::Mod::SetConfigHandler module records (name -> dynamic)
from Msg::ManifestOption events, then handles incoming setconfig
CommandRequests: it validates the named option is registered and
dynamic, and re-raises a fresh Msg::Option on the bus, so existing
option handlers re-apply the new value transparently.
Because Msg::Option is now re-emitted at runtime (not only during init),
subscribers must filter by name and tolerate post-init arrival.
AmountSettingsHandler gains an `if (!settings) return` guard: it moves
`settings` away at EndOfOptions, so a later Msg::Option for an unrelated
name must be dropped -- this also fixes a latent assert(settings) crash
that any post-EndOfOptions Msg::Option would have tripped.
Contract documented in SetConfigHandler.hpp: lightningd delivers
Int/Bool/Flag option values as JSON primitives at startup but as JSON
strings at setconfig time, so dynamic-option handlers must accept both
Jsmn shapes.
