Added configuration manager to PhotinoBlazorAppBuilder

[En3Tho]

I was very surprised PhotinoBlazorAppBuilder didn't have a ConfigurationManager considering many other builders have one.
E.g.:

public sealed class MauiAppBuilder
{
    /// <summary>
    /// A collection of configuration providers for the application to compose. This is useful for adding new configuration sources and providers.
    /// </summary>
    public ConfigurationManager Configuration { get; }
}

public sealed class WebApplicationBuilder
{
    /// <summary>
    /// Initial configuration sources to be added to the <see cref="HostApplicationBuilder.Configuration"/>. These sources can influence
    /// the <see cref="HostApplicationBuilder.Environment"/> through the use of <see cref="HostDefaults"/> keys. Disposing the built
    /// <see cref="IHost"/> disposes the <see cref="ConfigurationManager"/>.
    /// </summary>
    public ConfigurationManager? Configuration { get; set; }
}

public sealed class WebAssemblyHostBuilder // this one is a special one but it has one too
{
    /// <summary>
    /// Gets an <see cref="WebAssemblyHostConfiguration"/> that can be used to customize the application's
    /// configuration sources and read configuration attributes.
    /// </summary>
    public WebAssemblyHostConfiguration Configuration { get; }
}

This also will help a little to implement new IHostApplicationBuilder interface introduced in .Net 8 (dotnet/runtime#86974)

[ottodobretsberger]

Would you mind elaborating on a specific use case where this will come in handy?

[En3Tho]

This is a standard way in AspNetCore/Blazor/modern .Net hosting model to get configuration values. e.g. connection strings, urls, secrets etc. This improves builder so that in can be used in a similar way to WebAssemblyBuilder or MauiBuilder. Otherwise it just has to be created manually.

I think these should be added too to match the new IHostApplicationBuilder interface (I belive all .Net HostApplicationBuilders will inherit this one)

Microsoft.Extensions.Hosting.IHostEnvironment Environment { get; }
Microsoft.Extensions.Logging.ILoggingBuilder Logging { get; }

This just enables more code sharing between different hosting models while having less friction