feat: add HttpClientFactory option to HttpOptions for custom HttpClient configuration

[niimima]

Summary

Add HttpClientFactory to HttpOptions to allow callers to provide a custom HttpClient instance.

This enables advanced configuration scenarios such as:

• custom message handlers
• default headers
• proxy settings
• custom timeout policies
• integration with IHttpClientFactory

Changes

• Add Func<HttpClient>? HttpClientFactory property to HttpOptions
• Mark the property with [JsonIgnore] to prevent serialization
• Provide XML documentation for the new option

Motivation

In some environments, users need to control how HttpClient instances are created
(e.g., using IHttpClientFactory, custom handlers, or proxy settings).

This change allows consumers of the library to inject their own HttpClient
without modifying the internal HTTP pipeline.

Example

var options = new HttpOptions
{
    HttpClientFactory = () =>
    {
        var handler = new HttpClientHandler
        {
            Proxy = new WebProxy("http://proxy.example.com:8080"),
            UseProxy = true
        };

        return new HttpClient(handler);
    }
};

Compatibility

This change is backward compatible since the new property is optional.

[shivvaam0001]

Hi @niimima

Thank you for your contribution. Please check the failing checks and update the same.
Thanks!

[niimima]

@shivvaam0001
Thanks for the feedback!

I've fixed the failing checks and updated the PR.
I noticed this change touches an auto-generated file, but I couldn't find the source/template. Please let me know if you'd prefer the change in a different location.

Please let me know if anything else needs to be addressed.