Spec: XCQ executor API

[xlc]

Define the interface for XCQ executor to communicate with XCQ program

#[polkavm_derive::polkavm_import]
extern "C" {
    fn query(id: ExtendionId, method: u8, args: &[u8]) -> Vec<u8>;
}

#[polkavm_derive::polkavm_export]
extern "C" fn extensions() -> Vec<ExtensionId> {
	// return the extensions list
}

#[polkavm_derive::polkavm_export]
extern "C" fn execute(args: &[u8]) -> Vec<u8> {
	// The program body
}