feat(langchain): v1 scaffolding (#32166)

This PR adds scaffolding for langchain 1.0 entry package.

Most contents have been removed. 

Currently remaining entrypoints for:

* chat models
* embedding models
* memory -> trimming messages, filtering messages and counting tokens
[we may remove this]
* prompts -> we may remove some prompts
* storage: primarily to support cache backed embeddings, may remove the
kv store
* tools -> report tool primitives

Things to be added:

* Selected agent implementations
* Selected workflows
* Common primitives: messages, Document
* Primitives for type hinting: BaseChatModel, BaseEmbeddings
* Selected retrievers
* Selected text splitters

Things to be removed:

* Globals needs to be removed (needs an update in langchain core)


Todos: 

* TBD indexing api (requires sqlalchemy which we don't want as a
dependency)
* Be explicit about public/private interfaces (e.g., likely rename
chat_models.base.py to something more internal)
* Remove dockerfiles
* Update module doc-strings and README.md

libs/langchain_v1/tests/unit_tests/tools/test_imports.py

@@ -0,0 +1,14 @@
+from langchain import tools
+
+EXPECTED_ALL = {
+    "BaseTool",
+    "InjectedToolArg",
+    "InjectedToolCallId",
+    "Tool",
+    "ToolException",
+    "tool",
+}
+
+
+def test_all_imports() -> None:
+    assert set(tools.__all__) == EXPECTED_ALL