services.AddHttpClient("a业务").ConfigurePrimaryHttpMessageHandler(...a业务证书)
services.AddHttpClient("b业务").ConfigurePrimaryHttpMessageHandler(...b业务证书)
ServiceProvider.GetService<IHttpClientFactory>().CreateClient("a业务")....

class MyClass : IPostConfigureOptions<HttpClientFactoryOptions>
        {
            public void PostConfigure(string name, HttpClientFactoryOptions options)
                => options.HttpMessageHandlerBuilderActions.Add(p => p.PrimaryHandler = get_cert_handler_by_name(name));
        }
//注册这个服务
services.AddSingleton<Microsoft.Extensions.Options.IPostConfigureOptions<Microsoft.Extensions.Http.HttpClientFactoryOptions>, MyClass>();

//Microsoft.Extensions.Http.DefaultHttpClientFactory
public HttpClient CreateClient(string name)
{
	HttpClient httpClient = new HttpClient(this.CreateHandler(name), disposeHandler: false);
	return httpClient;
}
public HttpMessageHandler CreateHandler(string name)
{
	ActiveHandlerTrackingEntry value = this._activeHandlers.GetOrAdd(name, this._entryFactory).Value; 
        //_entryFactory可以直接理解为是CreateHandlerEntry方法.它真实的类型是Lazy<>(CreateHandlerEntry,LazyThreadSafetyMode.ExecutionAndPublication)的, 也就是并发安全的调用CreateHandlerEntry.
	return value.Handler;
}
internal ActiveHandlerTrackingEntry CreateHandlerEntry(string name)
{
        HttpClientFactoryOptions options = this._optionsMonitor.Get(name);
	HttpMessageHandlerBuilder requiredService = provider.GetRequiredService<HttpMessageHandlerBuilder>();
	requiredService.Name = name;
	Action<HttpMessageHandlerBuilder> action = Configure; // 扩展点二 HttpClientFactoryOptions.HttpMessageHandlerBuilderActions
	for (int num = this._filters.Length - 1; num >= 0; num--)
	{
		action = this._filters[num].Configure(action); //扩展点一 _filters(构造函数传入的IEnumerable<IHttpMessageHandlerBuilderFilter> filters).
	}
	action(requiredService);
	LifetimeTrackingHttpMessageHandler handler = new LifetimeTrackingHttpMessageHandler(requiredService.Build());
	return new ActiveHandlerTrackingEntry(name, handler, serviceScope, options.HandlerLifetime);

	void Configure(HttpMessageHandlerBuilder b)
	{
		for (int i = 0; i < options.HttpMessageHandlerBuilderActions.Count; i++)
		{
			options.HttpMessageHandlerBuilderActions[i](b);
		}
	}
}

class MyHttpClientHandlerFilter : IHttpMessageHandlerBuilderFilter
{
    public Action<HttpMessageHandlerBuilder> Configure(Action<HttpMessageHandlerBuilder> next)
    {
        void Configure(HttpMessageHandlerBuilder builder)
        {
            next(builder); //一开始就调用next, 这样我们的整个HandlerBuilder的执行顺序就是依次call _filters, 最后call options.HttpMessageHandlerBuilderActions(扩展点二).

            if (builder.Name.StartsWith("CallbackProviderSide-")) //我们可以为这类业务统一加一个前缀做区别, 这样就不会影响其他的HttpClient对象了.
            {
                //builder.PrimaryHandler= your custom handler. 参考官方文档的实现.
            }
        }
        return Configure;
    }
}
//然后在DI容器中注入我们的filter.
ServiceCollection.AddSingleton<IHttpMessageHandlerBuilderFilter,MyHttpClientHandlerFilter>();

class MyHttpClientCustomConfigure : IPostConfigureOptions<HttpClientFactoryOptions>
{
    public void PostConfigure(string name, HttpClientFactoryOptions options)
    {
        if (name.StartsWith("CallbackProviderSide-")) //我们可以为这类业务统一加一个前缀做区别, 这样就不会影响其他的HttpClient对象了.
        {
            options.HttpMessageHandlerBuilderActions.Add(p =>
            {
                //p.PrimaryHandler= your custom handler. 参考官方文档的实现.
            });
        }
    }
}

//然后在DI容器中注入我们的这个配置扩展类.
ServiceCollection.AddSingleton<Microsoft.Extensions.Options.IPostConfigureOptions<Microsoft.Extensions.Http.HttpClientFactoryOptions>, MyHttpClientCustomConfigure>();

var factory = ServiceProvider.GetService<IHttpClientFactory>();
var httpClientForBaidu = factory.CreateClient("CallbackProviderSide-baidu");
var httpClientForCnblogs = factory.CreateClient("CallbackProviderSide-Cnblogs");