chore: Only put common stuff in the templated `default.nix`

[NickLarsenNZ]

note(non-blocking): We should only put common stuff in the templated default.nix. All other operator-specific stuff should not be templated and should live in the operator repositories. @NickLarsenNZ will look into that.

Originally posted by @Techassi in #455 (comment)

[NickLarsenNZ]

We currently have a bunch of operator-specific overrides here:

operator-templating/template/default.nix

Lines 6 to 36 in 2a7dc83

	defaultCrateOverrides = pkgs.defaultCrateOverrides // {
	prost-build = attrs: {
	buildInputs = [ pkgs.protobuf ];
	};
	tonic-reflection = attrs: {
	buildInputs = [ pkgs.rustfmt ];
	};
	csi-grpc = attrs: {
	nativeBuildInputs = [ pkgs.protobuf ];
	};
	stackable-secret-operator = attrs: {
	buildInputs = [ pkgs.protobuf pkgs.rustfmt ];
	};
	krb5-sys = attrs: {
	nativeBuildInputs = [ pkgs.pkg-config ];
	buildInputs = [ pkgs.krb5 ];
	LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
	# Clang's resource directory is located at ${pkgs.clang.cc.lib}/lib/clang/<version>.
	# Starting with Clang 16, only the major version is used for the resource directory,
	# whereas the full version was used in prior Clang versions (see
	# https://github.com/llvm/llvm-project/commit/e1b88c8a09be25b86b13f98755a9bd744b4dbf14).
	# The clang wrapper ${pkgs.clang} provides a symlink to the resource directory, which
	# we use instead.
	BINDGEN_EXTRA_CLANG_ARGS = "-I${pkgs.glibc.dev}/include -I${pkgs.clang}/resource-root/include";
	};
	libgssapi-sys = attrs: {
	buildInputs = [ pkgs.krb5 ];
	LIBCLANG_PATH = "${pkgs.libclang.lib}/lib";
	BINDGEN_EXTRA_CLANG_ARGS = "-I${pkgs.glibc.dev}/include -I${pkgs.clang}/resource-root/include";
	};
	};

Hopefully there is a way we can just include them from another non-templated file (eg: overides.nix), and if it doesn't exist, default to {}.