[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/client/src/api_response.rs

@@ -0,0 +1,18 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize)]
+pub struct UpcheckApiResponse {
+    pub status: String,
+}
+
+/// Contains the response to the `get_keys` API.
+#[derive(Deserialize, Serialize)]
+pub struct KeysApiResponse {
+    pub keys: Vec<String>,
+}
+
+/// Contains the response to the `sign_message` API.
+#[derive(Deserialize, Serialize)]
+pub struct SignatureApiResponse {
+    pub signature: String,
+}