feature!: support for setting HTTP User-Agent header (#157)

* feature!: support for setting HTTP User-Agent header

* fix lint

* fix e2e

* refactor: move http.go to internal/util/useragent package and rename functions to Get and Set

* Move packages and use attest version

internal/useragent/useragent.go

@@ -0,0 +1,31 @@
+package useragent
+
+import (
+	"context"
+
+	"github.com/docker/attest/internal/version"
+)
+
+type userAgentKeyType string
+
+const (
+	userAgentKey     userAgentKeyType = "attest-user-agent"
+	defaultUserAgent string           = "attest/unknown (docker)"
+)
+
+func Set(ctx context.Context, userAgent string) context.Context {
+	return context.WithValue(ctx, userAgentKey, userAgent)
+}
+
+// Get retrieves the HTTP user agent from the context.
+func Get(ctx context.Context) string {
+	if ua, ok := ctx.Value(userAgentKey).(string); ok {
+		return ua
+	}
+	version, err := version.Get()
+	if err != nil || version == nil {
+		return defaultUserAgent
+	}
+
+	return "attest/" + version.String() + " (docker)"
+}