Add (podman {image,manifest} push --sign-by-sigstore=param-file.yaml)

(podman push) and (podman manifest push) 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/google.golang.org/appengine/internal/main_common.go

@@ -0,0 +1,7 @@
+package internal
+
+// MainPath stores the file path of the main package. On App Engine Standard
+// using Go version 1.9 and below, this will be unset. On App Engine Flex and
+// App Engine Standard second-gen (Go 1.11 and above), this will be the
+// filepath to package main.
+var MainPath string