plugin command's `options` argument expecting flattened options causes problem in generated rust/protobuf code

[daywalker90]

Looking at the schema in plugin.json you would expect this to work:

lightning-cli plugin subcommand=start plugin=/tmp/l1/reckless/my-plugin options='["my-option=my-value"]'

but this gives a "unknown parameter options" error. The c code actually doesn't look for an options argument but expects the options flattened like this:

lightning-cli plugin subcommand=start plugin=/tmp/l1/reckless/my-plugin my-option=my-value

While this is fine on the CLI, it does not work directly in our generated rust/protobuf code. It could be fixed in the rust code with a flatten annotation but there is no such thing for protobuf files. So i recommend changing the c code to actually look for an options argument. This would also better fit the json schema imo.