Bring back support for JwtUtils.IssueUserJWT

[aachinfiev]

Proposed change

In v1 client there was a JwtUtility which had IssueUserJWT function that can be handy when this things have to be generated on the fly. v2 client doesn't have this feature any more.

After discussing with @mtmk he mentioned that there is a plan to build a separate package to support such functionality in the future. Creating this ticket to keep track of this.

Original code: https://github.com/nats-io/nats.net/blob/a85c85692021c72ffcfbb0d93a799366afa35669/src/NATS.Client/Internals/JwtUtils.cs#L25

Use case

A system can have its own user set for which there are no pre-generated credentials. There is a need to generate user JWT for NATS authentication.

Contribution

No response