enable build with nix flakes

By default runs the bitcoin version

```shell
$ nix run .
Config { log: StdErrLog { verbosity: Error, quiet: false, show_level: true, timestamp: Off, modules: [], writer: "stderr", color_choice: Auto, show_module_names: false }, network_type: Bitcoin, db_path: "./db/mainnet", daemon_dir: "/home/casatta/.bitcoin", blocks_dir: "/home/casatta/.bitcoin/blocks", daemon_rpc_addr: 127.0.0.1:8332, cookie: None, electrum_rpc_addr: 127.0.0.1:50001, http_addr: 127.0.0.1:3000, http_socket_file: None, monitoring_addr: 127.0.0.1:4224, jsonrpc_import: false, light_mode: false, address_search: false, index_unspendables: false, cors: None, precache_scripts: None, utxos_limit: 500, electrum_txs_limit: 500, electrum_banner: "Welcome to electrs-esplora 0.4.1", electrum_rpc_logging: None }
```

But it contains also a package with the liquid variant:

```shell
$ nix run .#blockstream-electrs-liquid -- --network liquid
Config { log: StdErrLog { verbosity: Error, quiet: false, show_level: true, timestamp: Off, modules: [], writer: "stderr", color_choice: Auto, show_module_names: false }, network_type: Liquid, db_path: "./db/liquid", daemon_dir: "/home/casatta/.bitcoin/liquidv1", blocks_dir: "/home/casatta/.bitcoin/liquidv1/blocks", daemon_rpc_addr: 127.0.0.1:7041, cookie: None, electrum_rpc_addr: 127.0.0.1:51000, http_addr: 127.0.0.1:3000, http_socket_file: None, monitoring_addr: 127.0.0.1:34224, jsonrpc_import: false, light_mode: false, address_search: false, index_unspendables: false, cors: None, precache_scripts: None, utxos_limit: 500, electrum_txs_limit: 500, electrum_banner: "Welcome to electrs-esplora 0.4.1", electrum_rpc_logging: None, parent_network: Bitcoin, asset_db_path: None }
```

.gitignore

@@ -4,3 +4,4 @@ target
 *.sublime*
 *~
 *.pyc
+result

flake.nix

@@ -0,0 +1,85 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+    rust-overlay = {
+      url = "github:oxalica/rust-overlay";
+      inputs = {
+        nixpkgs.follows = "nixpkgs";
+        flake-utils.follows = "flake-utils";
+      };
+    };
+    crane = {
+      url = "github:ipetkov/crane";
+      inputs = {
+        nixpkgs.follows = "nixpkgs";
+      };
+    };
+  };
+  outputs = { self, nixpkgs, flake-utils, rust-overlay, crane }:
+    flake-utils.lib.eachDefaultSystem
+      (system:
+        let
+          overlays = [ (import rust-overlay) ];
+          pkgs = import nixpkgs {
+            inherit system overlays;
+          };
+          rustToolchain = pkgs.pkgsBuildHost.rust-bin.fromRustupToolchainFile ./rust-toolchain.toml;
+
+          craneLib = (crane.mkLib pkgs).overrideToolchain rustToolchain;
+
+          src = craneLib.cleanCargoSource ./.; 
+
+          nativeBuildInputs = with pkgs; [ rustToolchain clang ]; # required only at build time
+          buildInputs = with pkgs; [ ]; # also required at runtime
+
+          commonArgs = {
+            inherit src buildInputs nativeBuildInputs;
+            LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
+            ELEMENTSD_SKIP_DOWNLOAD = true;
+            BITCOIND_SKIP_DOWNLOAD = true;
+            ELECTRUMD_SKIP_DOWNLOAD = true;
+          };
+          cargoArtifacts = craneLib.buildDepsOnly commonArgs;
+          bin = craneLib.buildPackage (commonArgs // {
+            inherit cargoArtifacts;
+
+            # TODO: do testing by providing executables via *_EXEC env var for {bitcoin,elements,electrum}d
+            doCheck = false;
+          });
+          binLiquid = craneLib.buildPackage (commonArgs // {
+            inherit cargoArtifacts;
+            cargoExtraArgs = "--features liquid";
+
+            # TODO: do testing by providing executables via *_EXEC env var for {bitcoin,elements,electrum}d
+            doCheck = false;
+          });
+
+
+        in
+        with pkgs;
+        {
+          packages =
+            {
+              # that way we can build `bin` specifically,
+              # but it's also the default.
+              inherit bin;
+              default = bin;
+            };
+
+          apps."blockstream-electrs-liquid" = {
+            type = "app";
+            program = "${binLiquid}/bin/electrs";
+          };
+          apps."blockstream-electrs" = {
+            type = "app";
+            program = "${bin}/bin/electrs";
+          };
+
+
+          devShells.default = mkShell {
+            inputsFrom = [ bin ];
+          };
+        }
+      );
+}

rust-toolchain.toml

@@ -0,0 +1,2 @@
+[toolchain]
+channel = "1.75.0"