[API Proposal]: Allow to configure the login path in Identity

## Background and Motivation

Currently it doesn't seem to be an easy and built-in way to customise the login url used by the default authentication scheme in `Identity` apart from reconfiguring the scheme itself.

https://github.com/dotnet/aspnetcore/blob/240377059ec25b4d9d86d4188a26722e55edc5a1/src/Identity/Core/src/IdentityServiceCollectionExtensions.cs#L61-L61

## Proposed API

```diff
namespace Microsoft.AspNetCore.Identity;

public partial class SignInOptions
{
+ public string LoginPath { get; set; } = "/Account/Login";
}
```

## Usage Examples

```c#
builder.Services.Configure(options =>
{
options.SignIn.LoginPath = "/login";
});
```

## Risks

Nothing I can see now as the behaviour will remain the same thanks to the default value which is the same as the one used today.