Add DocstoreFn - lookup doc via arbitrary function (#3760)

This **partially** addresses
https://github.com/hwchase17/langchain/issues/1524, but it's also useful
for some of our use cases.

This `DocstoreFn` allows to lookup a document given a function that
accepts the `search` string without the need to implement a custom
`Docstore`.

This could be useful when:
* you don't want to implement a `Docstore` just to provide a custom
`search`
 * it's expensive to construct an `InMemoryDocstore`/dict
 * you retrieve documents from remote sources
 * you just want to reuse existing objects

tests/unit_tests/docstore/test_arbitrary_fn.py

@@ -0,0 +1,12 @@
+from langchain.docstore.arbitrary_fn import DocstoreFn
+from langchain.schema import Document
+
+
+def test_document_found() -> None:
+    # we use a dict here for simiplicity, but this could be any function
+    # including a remote lookup
+    dummy_dict = {"foo": Document(page_content="bar")}
+    docstore = DocstoreFn(lambda x: dummy_dict[x])
+    output = docstore.search("foo")
+    assert isinstance(output, Document)
+    assert output.page_content == "bar"