m/n/build: implement new fsspec infrastructure

This makes the node_initramfs and erofs_image use the new common fsspec
infrastructure. It also adds the fsspecs attribute to both which can
later be used to add arbitrary fsspecs.

Change-Id: I384e04712c0a70f82c5c975911cbb1d0d5e6cabc
Reviewed-on: https://review.monogon.dev/c/monogon/+/530
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/node/build/fsspec/BUILD.bazel b/metropolis/node/build/fsspec/BUILD.bazel
index 3a65d97..e0ce66d 100644
--- a/metropolis/node/build/fsspec/BUILD.bazel
+++ b/metropolis/node/build/fsspec/BUILD.bazel
@@ -10,9 +10,11 @@
 
 go_library(
     name = "go_default_library",
+    srcs = ["utils.go"],
     embed = [":fsspec_go_proto"],
     importpath = "source.monogon.dev/metropolis/node/build/fsspec",
     visibility = ["//visibility:public"],
+    deps = ["@com_github_golang_protobuf//proto:go_default_library"],
 )
 
 go_proto_library(