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

attestation/example_attestation_manifest_test.go

@@ -82,7 +82,8 @@ func ExampleManifest() {
 	if err != nil {
 		panic(err)
 	}
-	err = oci.SaveImagesNoTag(artifacts, output)
+	ctx := context.Background()
+	err = oci.SaveImagesNoTag(ctx, artifacts, output)
 	if err != nil {
 		panic(err)
 	}