nixos/bcache: add a `boot.bcache.enable` kill switch

My system does not use `bcache` and I sould prever my `systemPackages`
not to have bcache tools.

The change does not change the default but proviced usual `enable` knob.

nixos/doc/manual/release-notes/rl-2311.section.md

···
 
 - Certificate generation via the `security.acme` now limits the concurrent number of running certificate renewals and generation jobs, to avoid spiking resource usage when processing many certificates at once. The limit defaults to *5* and can be adjusted via `maxConcurrentRenewals`. Setting it to *0* disables the limits altogether.
 
+- New `boot.bcache.enable` (default enabled) allows completely removing `bcache` mount support.
+
 ## Nixpkgs internals {#sec-release-23.11-nixpkgs-internals}
 
 - The use of `sourceRoot = "source";`, `sourceRoot = "source/subdir";`, and similar lines in package derivations using the default `unpackPhase` is deprecated as it requires `unpackPhase` to always produce a directory named "source". Use `sourceRoot = src.name`, `sourceRoot = "${src.name}/subdir";`, or `setSourceRoot = "sourceRoot=$(echo */subdir)";` or similar instead.

nixos/modules/tasks/bcache.nix

···
-{ config, lib, pkgs, ... }:
-
-{
+{ config, lib, pkgs, ... }: let
+  cfg = config.boot.bcache;
+in {
+  options.boot.bcache.enable = lib.mkEnableOption (lib.mdDoc "bcache mount support") // {
+    default = true;
+    example = false;
+  };
   options.boot.initrd.services.bcache.enable = lib.mkEnableOption (lib.mdDoc "bcache support in the initrd") // {
     description = lib.mdDoc ''
       *This will only be used when systemd is used in stage 1.*
···
     '';
   };
 
-  config = {
+  config = lib.mkIf cfg.enable {
 
     environment.systemPackages = [ pkgs.bcache-tools ];