cmd/continuity: switch to google.golang.org/protobuf/proto

[thaJeztah]

• relates to https://github.com/containerd/continuity/pull/204
• relates to https://github.com/containerd/continuity/pull/255

The github.com/golang/protobuf/proto module is deprecated, and the main module already switched to google.golang.org/protobuf/proto in commit 74a016961cad4d635aeb6d4efb1bcc2268700d7a. This patch updates the cmd/continuity module to use the new module as well.

Before this patch:

$ cd cmd/continuity
$ go install
$ continuity build ./continuityfs > manifest.pb
$ continuity dump ./manifest.pb
resource: <
  path: "/fuse.go"
  uid: 501
  gid: 20
  mode: 420
  size: 7464
  digest: "sha256:3d86240dcc3ea8ec7ae85d6c4e0bf0be1385b5d1e38564dcec72a69b51fcf919"
>
resource: <
  path: "/provider.go"
  uid: 501
  gid: 20
  mode: 420
  size: 1728
  digest: "sha256:8e3da0fd4a8d5e454ff027d4948d4b116ddaeb5ccdd0e10af806a2ecf2599710"
>

With this patch:

$ cd cmd/continuity
$ go install
$ continuity build ./continuityfs > manifest.pb
$ continuity dump ./manifest.pb
resource:  {
  path:  "/fuse.go"
  uid:  501
  gid:  20
  mode:  420
  size:  7464
  digest:  "sha256:3d86240dcc3ea8ec7ae85d6c4e0bf0be1385b5d1e38564dcec72a69b51fcf919"
}
resource:  {
  path:  "/provider.go"
  uid:  501
  gid:  20
  mode:  420
  size:  1728
  digest:  "sha256:8e3da0fd4a8d5e454ff027d4948d4b116ddaeb5ccdd0e10af806a2ecf2599710"
}