[Remote signer] Add signer consumer lib (#1763)

Adds a library `common/remote_signer_consumer`

testing/remote_signer_test/src/mock.rs

@@ -0,0 +1,20 @@
+use crate::*;
+use httpmock::{Method::POST, MockServer};
+use tokio::time::Duration;
+
+pub fn set_up_mock_server(status: u16, body: &str) -> MockServer {
+    set_up_mock_server_with_timeout(status, body, 0)
+}
+
+pub fn set_up_mock_server_with_timeout(status: u16, body: &str, delay: u64) -> MockServer {
+    let server = MockServer::start();
+
+    server.mock(|when, then| {
+        when.method(POST).path(format!("/sign/{}", PUBLIC_KEY_1));
+        then.status(status)
+            .delay(Duration::from_secs(delay))
+            .body(body);
+    });
+
+    server
+}