chore(nix-module): improvements

[dpc]

• Ability to source the bitcoin rpc secret from a file (useful if you're using nix-bitcoin, or agenix, etc.)
• Ability to set user & group for the daemon.
• Cleanup.

Corresponding configuration.nix change (when using nix-bitcoin):

+  users.extraUsers.fedimintd-somefed.extraGroups = [ "bitcoinrpc-public" ];

   services.fedimintd."somefed" = {
     enable = true;
     package = pkgs.fedimintd;
+    extraEnvironment = {
+      "RUST_LOG" = "info";
+      "RUST_BACKTRACE" = "1";
+    };
     api = {
       address = "wss://${fmApiFqdn}/ws/";
       bind = "127.0.0.1";
@@ -133,7 +126,8 @@ in
     bitcoin = {
       network = "bitcoin";
       rpc = {
-        address = "http://bitcoin:${btcPass}@127.0.0.1:8332";
+        address = "http://bitcoin@127.0.0.1:8332";
+        secretFile = "/etc/nix-bitcoin-secrets/bitcoin-rpcpassword-public";
       };
     };
   };

[elsirion]

Although, since this is to be backported I'd like to avoid churn. @dpc do you want to change anything?

[dpc]

I hope no changes will be needed (some changes probably will needed anyway :D).

[fedimint-backports]

Successfully created backport PR for releases/v0.3:

• 5196