Add macro that registers guest policy endpoints

[ereslibre]

When writing a policy with this SDK, the policy has to implement some boilerplate:

#[no_mangle]
pub extern "C" fn wapc_init() {
    register_function("validate", validate);
    register_function("validate_settings", validate_settings::<Settings>);
    register_function("protocol_version", protocol_version_guest);
}

Add a macro that generates this code automatically, something like:

guest_entrypoints!(
  validate => validate,
  validate_settings => Settings,
)

The protocol_version will be automatically set by the macro to a generated function that returns the default ProtocolVersion with the current protocol version of the SDK being used by the policy.

Naming can likely be improved :)