[Remote signer] Fold signer into Lighthouse repository (#1852)

The remote signer relies on the `types` and `crypto/bls` crates from Lighthouse. Moreover, a number of tests of the remote signer consumption of LH leverages this very signer, making any important update a potential dependency nightmare.

Co-authored-by: Paul Hauner <paul@paulhauner.com>

remote_signer/tests/upcheck.rs

@@ -0,0 +1,16 @@
+mod upcheck {
+    use helpers::*;
+
+    #[test]
+    fn happy_path() {
+        let (test_signer, _tmp_dir) = set_up_api_test_signer_raw_dir();
+
+        let url = format!("{}/upcheck", test_signer.address);
+
+        let resp = http_get(&url);
+        assert_eq!(resp.status, 200);
+        assert_eq!(resp.json["status"], "OK");
+
+        test_signer.shutdown();
+    }
+}