build: add generating compat.json (#9700)

This is to add the generation of `compat.json` as a release artifact. It
describes the runtime requirements of the release in question. The next
step is to have the upgrade server use this information to filter
releases provided to clients. This is per the discussion in #9656

---------

Co-authored-by: Ross Smith II <ross@smithii.com>

lib/upgrade/upgrade_common.go

@@ -38,6 +38,13 @@ type Asset struct {
 	BrowserURL string `json:"browser_download_url,omitempty"`
 }
 
+// ReleaseCompatibility defines the structure of compat.json, which is
+// included with each elease.
+type ReleaseCompatibility struct {
+	Runtime      string            `json:"runtime,omitempty"`
+	Requirements map[string]string `json:"requirements,omitempty"`
+}
+
 var (
 	ErrNoReleaseDownload  = errors.New("couldn't find a release to download")
 	ErrNoVersionToSelect  = errors.New("no version to select")