Add boilerplate for key server APIs (#1196)

Also add a README which outilnes how things will work.

keyserver/api/api.go

@@ -0,0 +1,49 @@
+// Copyright 2020 The Matrix.org Foundation C.I.C.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package api
+
+import "context"
+
+type KeyInternalAPI interface {
+	PerformUploadKeys(ctx context.Context, req *PerformUploadKeysRequest, res *PerformUploadKeysResponse)
+	PerformClaimKeys(ctx context.Context, req *PerformClaimKeysRequest, res *PerformClaimKeysResponse)
+	QueryKeys(ctx context.Context, req *QueryKeysRequest, res *QueryKeysResponse)
+}
+
+// KeyError is returned if there was a problem performing/querying the server
+type KeyError struct {
+	Error string
+}
+
+type PerformUploadKeysRequest struct {
+}
+
+type PerformUploadKeysResponse struct {
+	Error *KeyError
+}
+
+type PerformClaimKeysRequest struct {
+}
+
+type PerformClaimKeysResponse struct {
+	Error *KeyError
+}
+
+type QueryKeysRequest struct {
+}
+
+type QueryKeysResponse struct {
+	Error *KeyError
+}