Might fix #28620
This PR only modifies the behavior of loadtxoutset to require a mandatory “start” string as a first parameter.
This change will allow to implement “status”, “abort” or similar future parameters in follow-ups, without breaking the current RPC protocol with incompatible changes (if this were to be merged in the current release cycle)
Also, it is implemented in a minimalist way, to make it easy to review.