[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/backend/src/error.rs

@@ -0,0 +1,46 @@
+#[derive(Debug)]
+pub enum BackendError {
+    /// Parameter is not a hexadecimal representation of a BLS public key.
+    InvalidPublicKey(String),
+
+    /// Retrieved value is not a hexadecimal representation of a BLS secret key.
+    InvalidSecretKey(String),
+
+    /// Public and Secret key won't match.
+    KeyMismatch(String),
+
+    /// Item requested by its public key is not found.
+    KeyNotFound(String),
+
+    /// Errors from the storage medium.
+    ///
+    /// When converted from `std::io::Error`, stores `std::io::ErrorKind`
+    /// and `std::io::Error` both formatted to string.
+    StorageError(String, String),
+}
+
+impl std::fmt::Display for BackendError {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        match self {
+            BackendError::InvalidPublicKey(e) => write!(f, "Invalid public key: {}", e),
+
+            // Feed it with the public key value used to retrieve it.
+            BackendError::InvalidSecretKey(e) => write!(f, "Invalid secret key: {}", e),
+
+            // Feed it with the public key value used to retrieve it.
+            BackendError::KeyMismatch(e) => write!(f, "Key mismatch: {}", e),
+
+            BackendError::KeyNotFound(e) => write!(f, "Key not found: {}", e),
+
+            // Only outputs to string the first component of the tuple, accounting
+            // for potential differences on error displays between OS distributions.
+            BackendError::StorageError(e, _) => write!(f, "Storage error: {}", e),
+        }
+    }
+}
+
+impl From<std::io::Error> for BackendError {
+    fn from(e: std::io::Error) -> BackendError {
+        BackendError::StorageError(format!("{:?}", e.kind()), format!("{}", e))
+    }
+}