Extend SDK with non-default verify implementation

[YaronWittenstein]

Right now, the SDK generates this verify code (svm_verify if to be precise)

 #[no_mangle]
pub extern "C" fn svm_verify() -> u32 {
    0
}

svm/crates/sdk-macros/src/template.rs

Lines 299 to 311 in 6edb73d

	fn export_verify_ast() -> TokenStream {
	quote! {
	#[no_mangle]
	pub extern "C" fn svm_verify() -> u32 {
	// TODO:
	// This is a temporary stub
	//
	// The Template Author will have to come up with
	// his own implementation for `svm_verify`
	0
	}
	}
	}

The Simple Coin Iteration #2 is about being able to implement meaningful verify that will implement algorithms such as Single-Sig or MultiSig.

• Introduce a new #[verify] proc-macro attribute. For references look at the current #[endpoint] and #[ctor].
• Enforce only one usage of #[verify] within a Template.
• In case no #[verify] usage is being used - we can decide to fall back to the current stub implementation for now.