da interface: add max blob size method

proto/da/da.proto

@@ -3,6 +3,9 @@ package da;
 
 // DAService is the protobuf service definition for interaction with Data Availability layers.
 service DAService {
+    // Config initialises the client
+    rpc Config(ConfigRequest) returns (ConfigResponse) {}
+
 	// Get returns Blob for each given ID, or an error.
 	rpc Get(GetRequest) returns (GetResponse) {}
 
@@ -39,6 +42,15 @@ message Proof {
 	bytes value = 1;
 }
 
+// ConfigRequest is the request type for the Config rpc method.
+message ConfigRequest {
+}
+
+// ConfigResponse is the response type for the Config rpc method.
+message ConfigResponse {
+    int64 max_blob_size = 1;
+}
+
 // GetRequest is the request type for the Get rpc method.
 message GetRequest {
 	repeated ID ids = 1;
@@ -89,4 +101,4 @@ message ValidateRequest {
 // ValidateResponse is the response type for the Validate rpc method.
 message ValidateResponse {
 	repeated bool results = 1;
-}
+}

proxy/server.go

@@ -24,6 +24,10 @@ type proxySrv struct {
 	target da.DA
 }
 
+func (p *proxySrv) Config(ctx context.Context, request *pbda.ConfigRequest) (*pbda.ConfigResponse, error) {
+	return &pbda.ConfigResponse{MaxBlobSize: 1024}, nil
+}
+
 func (p *proxySrv) Get(ctx context.Context, request *pbda.GetRequest) (*pbda.GetResponse, error) {
 	ids := idsPB2DA(request.Ids)
 	blobs, err := p.target.Get(ids)