allow management endpoints to be overwritten manually

[Erwinvandervalk]

What issue does this PR address?
In V3, you could extend the management endpoints by inheriting from the default functionality.

In V4, this is no longer possible as the default impl is internal. You can replace the endpoint entirely (via DI). But if you want to extend the functionality, this was a bit clunky:

        app.Use(async (c, n) =>
        {
            if (c.Request.Path.StartsWithSegments("/bff/login"))
            {
                loginCalled = true;
                await c.RequestServices.GetRequiredService<ILoginEndpoint>().ProcessRequestAsync(c);
                return;
            }
            await n();
        });

Make sure that this is possible:

        app.MapGet(Bff.BffOptions.LoginPath, c =>
        {
            loginCalled = true;
            return c.RequestServices.GetRequiredService<ILoginEndpoint>().ProcessRequestAsync(c);
        });

fixes: https://github.com/DuendeSoftware/products-private/issues/2285

Also, this PR marks the ISilentLoginEndpoint as obsolete. The implementation was already marked as obsolete, but this was an internal implementation.

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