feat: Add GetSpecSchema to the plugin spec (#20)

Author: candiduslynx

plugin/v3/plugin.proto

@@ -12,6 +12,10 @@ service Plugin {
   rpc GetName(GetName.Request) returns (GetName.Response);
   // Get the current version of the plugin
   rpc GetVersion(GetVersion.Request) returns (GetVersion.Response);
+  // Get plugin spec schema.
+  // This will allow validating the input even before calling Init.
+  // Should be called before Init.
+  rpc GetSpecSchema(GetSpecSchema.Request) returns (GetSpecSchema.Response);
   // Configure the plugin with the given credentials and mode
   rpc Init(Init.Request) returns (Init.Response);
   // Get all tables the source plugin supports. Must be called after Init
@@ -41,6 +45,15 @@ message GetVersion {
   }
 }
 
+message GetSpecSchema {
+  message Request {}
+  message Response {
+    // Should be a valid JSON schema for the plugin spec.
+    // See https://json-schema.org for more details.
+    optional string json_schema = 1;
+  }
+}
+
 message Init {
   message Request {
     bytes spec = 1; // Internal plugin-specific spec