Wire PPL conversion scalar functions with analytics-backend-datafusion (opensearch-project#21628)

* Wire PPL conversion scalar functions with analytics-backend-datafusion

Signed-off-by: Tanik Pansuriya <panbhai@amazon.com>

* Refactor conversion functions

Signed-off-by: Tanik Pansuriya <panbhai@amazon.com>

---------

Signed-off-by: Tanik Pansuriya <panbhai@amazon.com>

Author: tanik98

sandbox/libs/analytics-framework/src/main/java/org/opensearch/analytics/spi/ScalarFunction.java

@@ -102,6 +102,21 @@ public enum ScalarFunction {
     REX_EXTRACT_MULTI(Category.STRING, SqlKind.OTHER_FUNCTION),
     REX_OFFSET(Category.STRING, SqlKind.OTHER_FUNCTION),
 
+    // TODO: Frontend/lang-specific functions (NUM/AUTO/MEMK/MKTIME etc.) shouldn't
+    // live in the shared analytics-framework SPI — they couple the framework to PPL
+    // vocabulary. Replace with a registration-at-startup mechanism where each frontend
+    // supplies its own function set, keeping ScalarFunction to operators with
+    // cross-frontend semantics (CASE, COALESCE, comparisons, etc.).
+    NUM(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    AUTO(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    MEMK(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    RMCOMMA(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    RMUNIT(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    DUR2SEC(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    MSTIME(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    CTIME(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+    MKTIME(Category.SCALAR, SqlKind.OTHER_FUNCTION),
+
     // ── Cryptographic hash ─────────────────────────────────────────────
     // md5(x), sha1(x), sha2(x, bitLen) with bitLen ∈ {224,256,384,512}, crc32(x).
     MD5(Category.STRING, SqlKind.OTHER_FUNCTION),

sandbox/plugins/analytics-backend-datafusion/rust/src/udf/conversion/mod.rs

@@ -0,0 +1,24 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+//! Conversion-family UDFs grouped under one module.
+
+use datafusion::execution::context::SessionContext;
+
+pub mod numeric_conversion;
+pub mod time_conversion;
+pub mod tonumber;
+pub mod tostring;
+
+/// Register every conversion-family UDF on the given session context.
+pub fn register_all(ctx: &SessionContext) {
+    numeric_conversion::register_all(ctx);
+    time_conversion::register_all(ctx);
+    tonumber::register_all(ctx);
+    tostring::register_all(ctx);
+}