Introduce `ILogFormattable`

### Background and motivation

Currently LOGGEN036 requires that `[LoggerMessage]` parameter types "implement `ToString()`, `IConvertible`, `IFormattable`".
There's a problem with this:
All of these interface + `ToString()` are general; if I override `ToString()` for my custom type for the purpose of modifying how it ends up appearing in log messages, I'll incidentally be changing how it's included in **_any_** string.

Imagine a scenario in which I'd want only the last 12 characters in a given instance of a custom identifier struct to appear in log messages (but in full everywhere else).

### API Proposal

Introduce `Microsoft.Extensions.Logging.Abstractions.ILogFormattable`:

### API Usage

```csharp
[ValueObject] // using the Vogen library
public readonly partial struct MyId : ILogFormattable
{
public string FormatForLog() => Value.ToString()[^12..];
}
```

### Alternative Designs

_No response_

### Risks

_No response_