community: make SET allow_experimental_[engine]_index configurabe i…

…n vectorstores.clickhouse (langchain-ai#18107)

## Description & Issue
While following the official doc to use clickhouse as a vectorstore, I
found only the default `annoy` index is properly supported. But I want
to try another engine `usearch` for `annoy` is not properly supported on
ARM platforms.
Here is the settings I prefer:

``` python
settings = ClickhouseSettings(
    table="wiki_Ethereum",
    index_type="usearch",  # annoy by default
    index_param=[],
)
```
The above settings do not work for the command `set
allow_experimental_annoy_index=1` is hard-coded.
This PR will make sure the experimental feature follow the `index_type`
which is also consistent with Clickhouse's naming conventions.

libs/community/langchain_community/vectorstores/clickhouse.py

@@ -205,8 +205,8 @@ def __init__(
         )
         # Enable JSON type
         self.client.command("SET allow_experimental_object_type=1")
-        # Enable Annoy index
-        self.client.command("SET allow_experimental_annoy_index=1")
+        # Enable index
+        self.client.command(f"SET allow_experimental_{self.config.index_type}_index=1")
         self.client.command(self.schema)
 
     @property