Improve / Document creating a parameter with a dynamic default value

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your feature request related to a problem? Please describe the problem.

On aspirifriday today, we wanted to create a parameter value to represent a JWT signing key - a 32 byte base64 string.

But there was no easy way to provide our own default value generator. There is the (undocumented) `GenerateParameterDefault` class, but that would be very unlikely to generate a valid base64 string.

```cs
var jwtSigningKey = builder.AddParameter(
"jwt-signing-key",
new Base64ParameterDefault(32),
secret: true,
persist: true);

internal sealed class Base64ParameterDefault(int byteLength) : Aspire.Hosting.ApplicationModel.ParameterDefault
{
public int ByteLength { get; } = byteLength > 0
? byteLength
: throw new ArgumentOutOfRangeException(nameof(byteLength));

public override string GetDefaultValue()
{
return Convert.ToBase64String(System.Security.Cryptography.RandomNumberGenerator.GetBytes(ByteLength));
}

public override void WriteToManifest(Aspire.Hosting.Publishing.ManifestPublishingContext context)
{
context.Writer.WriteStartObject("generateBase64");
context.Writer.WriteNumber("byteLength", ByteLength);
context.Writer.WriteEndObject();
}
}
```

### Describe the solution you'd like

I'd like to see an easier way to provide a default value with little more than a generator function, something like the below. As well as some documentaiton on how to do so. (Which would likely also need to cover `GenerateParameterDefault`):

```cs
var jwtSigningKey = builder.AddParameter(
"jwt-signing-key",
() => (string)return CalculateDefaultValue(),
secret: true,
persist: true);
```
### Additional context

https://www.youtube.com/live/RGBK63vn3Rc?si=CRyNwxlB5POnBibs&t=5386

Start with the AddParameter API and the existing GenerateParameterDefault class mentioned in the issue, then review how custom parameter defaults are represented and documented. Done means a simpler generator-based default option is supported and the documentation explains both that option and GenerateParameterDefault.

Written by the indexing model from the issue text.