Add a HostedFileSearchTool

[lajjne]

Add HostedFileSearchTool similar to HostedWebSearchTool as mentioned in #6091.

For OpenAI, this can map to the file_search tool.

[kzu]

The alternative is allowing ResponseTools to be passed to AsIChatClient():

public static class OpenAIClientExtensions
{
    public static IChatClient AsIChatClient(this OpenAIResponseClient responseClient, params ResponseTool[] tools) =>
        new OpenAIResponseChatClient(responseClient, tools);
}

which then could be added to the response options before invoking the client.

[stephentoub]

Closing as a duplicate of #6378. This issue was opened first, but there's more discussion on the other one.