Make the NuidWriter class public

[Madgvox]

Proposed change

Move NuidWriter from NATS.Client.Core.Internal to NATS.Client.Core and make the class public instead of internal.

Use case

I use Nuids for identifiers throughout my application. The Nuid generator is public in v1 of the client, so it makes sense for it to be so in v2.

Contribution

I'm creating this issue for tracking purposes from the original discussion post, PR incoming shortly.