feat: add support for custom Hub endpoint

[sy1995]

Description

This PR adds support for custom Hub endpoints to improve network reliability. Users can now specify a custom endpoint (e.g., https://hf-mirror.com) when loading a model, which helps address network quality issues.

Changes

• Added a new loadContainer method in ModelFactory extension.
• Updated documentation to reflect the new functionality.
• Default endpoint is set to https://hf-mirror.com.

[davidkoski]

I don't think we should add a new method like this -- the non-default parameters will match the other method signatures and this may be hard to call.

I think a new property could be added to the ModelFactory protocol:

    var defaultHubApi: HubApi { get set }

and then the default implementations of these methods could be reworked like this:

    public func loadContainer(
        hub: HubApi? = nil, configuration: ModelConfiguration,
        progressHandler: @Sendable @escaping (Progress) -> Void = { _ in }
    ) async throws -> ModelContainer {
        try await _loadContainer(
            hub: hub ?? defaultHubApi, configuration: configuration, progressHandler: progressHandler)
    }

Then the caller can set a new default HubApi if they want.

[davidkoski]

Also, having access to mirrors sounds great but I think we should keep the default as it was. We can add an argument in the command line tools but I think app developers should decide if they want this specific mirror or not.