[BUG] FlyteRemote for_endpoint method has config_file parameter incorrectly typed

### Describe the bug

Initializing a UnionRemote object like so: `UnionRemote.for_endpoint(endpoint=UNION_ENDPOINT)` gives a Pyright typing error:

`Argument of type "None" cannot be assigned to parameter "config_file" of type "str | ConfigFile" in function "for_endpoint"`

Looking at the source code ([here](https://github.com/flyteorg/flytekit/blob/c95a3df85ccbb6552cc7d315a4fb2cd77193d54f/flytekit/remote/remote.py#L3034)), it appears the `config_file` parameter is not typed as Optional, but defaults to `None`, causing the typing error.

### Expected behavior

I don't expect there to be a typing error upon UnionRemote initialization

### Additional context to reproduce

_No response_

### Screenshots

_No response_

### Are you sure this issue hasn't been raised already?

- [x] Yes

### Have you read the Code of Conduct?

- [x] Yes