Add support for Fulcio and Rekor, and --sign-by-sigstore=param-file

(skopeo copy) and (skopeo sync) now support --sign-by-sigstore=param-file,
using the containers-sigstore-signing-params.yaml(5) file format.

That notably adds support for Fulcio and Rekor signing.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

vendor/github.com/go-openapi/validate/update-fixtures.sh

@@ -0,0 +1,15 @@
+#!/bin/bash 
+
+set -eu -o pipefail
+dir=$(git rev-parse --show-toplevel)
+scratch=$(mktemp -d -t tmp.XXXXXXXXXX)
+
+function finish {
+  rm -rf "$scratch"
+}
+trap finish EXIT SIGHUP SIGINT SIGTERM
+
+cd "$scratch"
+git clone https://github.com/json-schema-org/JSON-Schema-Test-Suite Suite
+cp -r Suite/tests/draft4/* "$dir/fixtures/jsonschema_suite"
+cp -a Suite/remotes "$dir/fixtures/jsonschema_suite"