feature: add sample user define aggregate function

tests/script/sh/squares_sum.c

+#include <string.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <math.h>
+
+#include "taosudf.h"
+
+DLL_EXPORT int32_t squares_sum_init() {
+  return 0;
+}
+
+DLL_EXPORT int32_t squares_sum_destroy() {
+  return 0;
+}
+
+DLL_EXPORT int32_t squares_sum_start(SUdfInterBuf *buf) {
+  *(int64_t*)(buf->buf) = 0;
+  buf->bufLen = sizeof(double);
+  buf->numOfResult = 0;
+  return 0;
+}
+
+DLL_EXPORT int32_t squares_sum(SUdfDataBlock* block, SUdfInterBuf *interBuf, SUdfInterBuf *newInterBuf) {
+  double sumSquares = *(double*)interBuf->buf;
+  int8_t numNotNull = 0;
+  for (int32_t i = 0; i < block->numOfCols; ++i) {
+    SUdfColumn* col = block->udfCols[i];
+    if (!(col->colMeta.type == TSDB_DATA_TYPE_INT || 
+          col->colMeta.type == TSDB_DATA_TYPE_DOUBLE)) {
+      return TSDB_CODE_UDF_INVALID_INPUT;
+    }
+  }
+  for (int32_t i = 0; i < block->numOfCols; ++i) {
+    for (int32_t j = 0; j < block->numOfRows; ++j) {
+      SUdfColumn* col = block->udfCols[i];
+      if (udfColDataIsNull(col, j)) {
+        continue;
+      }
+      switch (col->colMeta.type) {
+        case TSDB_DATA_TYPE_INT: {
+          char* cell = udfColDataGetData(col, j);
+          int32_t num = *(int32_t*)cell;
+          sumSquares += (double)num * num;
+          break;
+        }
+        case TSDB_DATA_TYPE_DOUBLE: {
+          char* cell = udfColDataGetData(col, j);
+          double num = *(double*)cell;
+          sumSquares += num * num;
+          break;
+        }
+        default: 
+          break;
+      }
+      ++numNotNull;
+    }
+  }
+
+  *(double*)(newInterBuf->buf) = sumSquares;
+  newInterBuf->bufLen = sizeof(double);
+
+  if (interBuf->numOfResult == 0 && numNotNull == 0) {
+    newInterBuf->numOfResult = 0;
+  } else {
+    newInterBuf->numOfResult = 1;
+  }
+  return 0;
+}
+
+DLL_EXPORT int32_t udf2_finish(SUdfInterBuf* buf, SUdfInterBuf *resultData) {
+  if (buf->numOfResult == 0) {
+    resultData->numOfResult = 0;
+    return 0;
+  }
+  double sumSquares = *(double*)(buf->buf);
+  *(double*)(resultData->buf) = sqrt(sumSquares);
+  resultData->bufLen = sizeof(double);
+  resultData->numOfResult = 1;
+  return 0;
+}