langchain[minor],community[minor]: Add async methods in BaseLoader (#16634)

Adds:
* methods `aload()` and `alazy_load()` to interface `BaseLoader`
* implementation for class `MergedDataLoader `
* support for class `BaseLoader` in async function `aindex()` with unit
tests

Note: this is compatible with existing `aload()` methods that some
loaders already had.

**Twitter handle:** @cbornet_

---------

Co-authored-by: Eugene Yurtsev <eugene@langchain.dev>

libs/community/langchain_community/document_loaders/merge.py

@@ -1,4 +1,4 @@
-from typing import Iterator, List
+from typing import AsyncIterator, Iterator, List
 
 from langchain_core.documents import Document
 
@@ -26,3 +26,9 @@ class MergedDataLoader(BaseLoader):
     def load(self) -> List[Document]:
         """Load docs."""
         return list(self.lazy_load())
+
+    async def alazy_load(self) -> AsyncIterator[Document]:
+        """Lazy load docs from each individual loader."""
+        for loader in self.loaders:
+            async for document in loader.alazy_load():
+                yield document