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/hashicorp/go-cleanhttp/doc.go

@@ -0,0 +1,20 @@
+// Package cleanhttp offers convenience utilities for acquiring "clean"
+// http.Transport and http.Client structs.
+//
+// Values set on http.DefaultClient and http.DefaultTransport affect all
+// callers. This can have detrimental effects, esepcially in TLS contexts,
+// where client or root certificates set to talk to multiple endpoints can end
+// up displacing each other, leading to hard-to-debug issues. This package
+// provides non-shared http.Client and http.Transport structs to ensure that
+// the configuration will not be overwritten by other parts of the application
+// or dependencies.
+//
+// The DefaultClient and DefaultTransport functions disable idle connections
+// and keepalives. Without ensuring that idle connections are closed before
+// garbage collection, short-term clients/transports can leak file descriptors,
+// eventually leading to "too many open files" errors. If you will be
+// connecting to the same hosts repeatedly from the same client, you can use
+// DefaultPooledClient to receive a client that has connection pooling
+// semantics similar to http.DefaultClient.
+//
+package cleanhttp