cos-extensions: vendor Go dependencies for 9999 live ebuild

Vendor Go module dependencies via `go mod vendor` so app-admin/extensions-manager
can build offline (-mod=vendor) as a cros-workon 9999 live ebuild without a
separate -deps.tar.xz distfiles archive.

BUG=b/561624065
TEST=CGO_ENABLED=0 go build -mod=vendor
RELEASE_NOTE=None

cos-patch: bug
Change-Id: I0b12bfc55ad51c88b7057e985a7063b9e1e2c344
Reviewed-on: https://cos-review.googlesource.com/c/cos/cos-extensions/+/208046
SLSA-Policy-Verified: SLSA Policy Verification Service <devtools-gerritcodereview-exitgate@google.com>
Tested-by: Miri Amarilio <mirilio@google.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
AI-Review: cloud-image-release <cloud-image-release@prod.google.com>