allow passing api key as env var (#841)

Allow passing API key as env var:
```shell
export LANCEDB_API_KEY=sh_123...
```

with this set, apiKey argument can omitted from `connect`
```js
    const db = await vectordb.connect({
        uri: "db://test-proj-01-ae8343",
        region: "us-east-1",
  })
```
```py
    db = lancedb.connect(
        uri="db://test-proj-01-ae8343",
        region="us-east-1",
    )
```

python/lancedb/__init__.py

@@ -12,6 +12,7 @@
 #  limitations under the License.
 
 import importlib.metadata
+import os
 from typing import Optional
 
 __version__ = importlib.metadata.version("lancedb")
@@ -39,6 +40,7 @@ def connect(
     api_key: str, optional
         If presented, connect to LanceDB cloud.
         Otherwise, connect to a database on file system or cloud storage.
+        Can be set via environment variable `LANCEDB_API_KEY`.
     region: str, default "us-east-1"
         The region to use for LanceDB Cloud.
     host_override: str, optional
@@ -66,6 +68,8 @@ def connect(
         A connection to a LanceDB database.
     """
     if isinstance(uri, str) and uri.startswith("db://"):
+        if api_key is None:
+            api_key = os.environ.get("LANCEDB_API_KEY")
         if api_key is None:
             raise ValueError(f"api_key is required to connected LanceDB cloud: {uri}")
         return RemoteDBConnection(uri, api_key, region, host_override)