api: rename /api/versions to versionsapi and /api/attestationcfig to attestationconfigapi (#1876)

* rename to attestationconfigapi + put client and fetcher inside pkg

* rename api/version to versionsapi and put fetcher + client inside pkg

* rename AttestationConfigAPIFetcher to Fetcher

internal/api/versionsapi/apiconstants.go

@@ -0,0 +1,22 @@
+/*
+Copyright (c) Edgeless Systems GmbH
+
+SPDX-License-Identifier: AGPL-3.0-only
+*/
+
+package versionsapi
+
+var (
+	// APIV1 is the v1 API version.
+	APIV1 = apiVersion{slug: "v1"}
+	// APIV2 is the v2 API version.
+	APIV2 = apiVersion{slug: "v2"}
+)
+
+type apiVersion struct {
+	slug string
+}
+
+func (v apiVersion) String() string {
+	return v.slug
+}