Add a non-logging way to initialise Sentry in generic host apps

### Description

Companion to #5245, which removes the ability to initialise the SDK from the logging integrations in v7.

In a generic-host app (`Host.CreateApplicationBuilder()`, worker services, etc.) the only way to initialise Sentry today is through the Microsoft.Extensions.Logging integration:

```csharp
var builder = Host.CreateApplicationBuilder();
builder.Logging.AddSentry(dsn); // initialises the SDK
```

There is no `UseSentry()` outside ASP.NET Core. That path also gives these users `appsettings.json` binding for free, because `AddSentry` registers `IOptions` and binds the `Sentry` configuration section.

Once #5245 lands, `builder.Logging.AddSentry()` only wires up the logger provider. Generic-host users would be left calling `SentrySdk.Init` and binding configuration into it by hand — a capability removed without a replacement.

### Proposal

Add a host-level, non-logging way to initialise Sentry for generic-host apps, e.g. an extension on `IHostApplicationBuilder` (and possibly `IHostBuilder`), analogous to `UseSentry()` for ASP.NET Core:

- initialises the SDK through the existing `ServiceCollectionExtensions.AddSentry` plumbing (the same path ASP.NET Core and MAUI use)
- binds the `Sentry` configuration section, so `appsettings.json` keeps working
- registers the logger provider, so that `builder.Logging.AddSentry()` is not also needed

`samples/Sentry.Samples.GenericHost` is the case to design against.

Open questions:
- which package it lives in — `Sentry.Extensions.Logging` doesn't currently reference `Microsoft.Extensions.Hosting.Abstractions`, so either it takes that dependency or this becomes a new `Sentry.Extensions.Hosting` package
- naming: `builder.UseSentry()` for consistency with ASP.NET Core and MAUI, or `builder.Services.AddSentry()`

This should ship in the same release as #5245 so the removal never goes out without the replacement.

Start with samples/Sentry.Samples.GenericHost and the existing ServiceCollectionExtensions.AddSentry plumbing, then compare the ASP.NET Core and MAUI initialization paths. Review #5245 and the package dependency options before settling the host API and package location. Done means generic-host apps can initialize Sentry, bind the Sentry configuration section, and register the logger provider without manually calling SentrySdk.Init or AddSentry().

Written by the indexing model from the issue text.