Add RFC to support custom schemas in native sidecar function registry

RFC-0021-Support-custom-schemas-in-native-sidecar-function-registry.md

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+# **RFC-0021 for Presto**
+
+## [Title] Support custom schemas in native sidecar function registry
+
+## Summary
+This feature enables namespace separation for C++ functions in Presto by allowing registration and querying of functions under different catalogs and schemas. It brings the native sidecar registry to feature parity with Java function namespace managers.
+
+## [Related Issues]
+- https://github.com/prestodb/presto/issues/25429
+
+## Background
+Currently, all functions (built-in and custom) were bundled together, preventing proper isolation and organization. Users could not namespace custom C++ functions separately from built-ins.
+
+## Design & Architecture
+The solution introduces catalog-based filtering:
+```mermaid
+graph TD
+    Coordinator[Presto Coordinator]
+    NamespaceManager1[NativeFunctionNamespaceManager - Catalog: mycatalog]
+    NamespaceManager2[NativeFunctionNamespaceManager - Catalog: native]
+    EndpointCatalog["GET /v1/functions/{catalog}"]
+    FilterCatalog["getFunctionsMetadata(catalog)"]
+    Registry[Velox Function Registry]
+    Coordinator --> NamespaceManager1
+    Coordinator --> NamespaceManager2
+    NamespaceManager1 -->|"/v1/functions/mycatalog"| EndpointCatalog
+    NamespaceManager2 -->|"/v1/functions/native"| EndpointCatalog
+    EndpointCatalog --> FilterCatalog
+    FilterCatalog --> Registry
+```
+- **C++ Backend:** Adds `/v1/functions/{catalog}` endpoint in `PrestoServer.cpp` that delegates to `getFunctionsMetadata(catalog)`.
+- **Filtering Logic:** In `FunctionMetadata.cpp`, functions are filtered by catalog prefix (from fully qualified name: `catalog.schema.function_name`).
+- **Java Plugin:** Each `NativeFunctionNamespaceManager` instance is bound to a catalog and queries only its catalog's functions from the sidecar.
+
+## API Specification
+- **GET /v1/functions**: Returns all functions (unfiltered).
+- **GET /v1/functions/{catalog}**: Returns only functions from the specified catalog.
+
+Example response for `/v1/functions/mycatalog`:
+```json
+{
+  "my_custom_function": [
+    {
+      "outputType": "varchar",
+      "paramTypes": ["varchar"],
+      "schema": "myschema",
+      "functionKind": "SCALAR",
+      "routineCharacteristics": {
+        "language": {"languageName": "CPP"},
+        "determinism": "DETERMINISTIC",
+        "nullCallClause": "RETURNS_NULL_ON_NULL_INPUT"
+      }
+    }
+  ]
+}
+```
+
+## Implementation Details
+- Functions are registered in Velox as `catalog.schema.function_name`.
+- Filtering is performed by splitting the function name and matching the catalog.
+
+## Usage Example
+Configure multiple function namespace managers:
+```properties
+# etc/function-namespace/mycatalog.properties
+function-namespace-manager.name=native
+function-implementation-type=CPP
+supported-function-languages=CPP
+```