Ability to reload Gateway options when running embedded

[Nintron27]

Proposed change

The ability to reload the gateway options (server.Options.gateway) when running the nats-server embedded.

Currently if you clone the Options, modify the settings in Gateway and then call server.ReloadOptions(opts) you are returned an error such as: config reload not supported for Gateway: <GATEWAY STRUCTS HERE>.

Use case

This could be useful for projects where new gateways can be created or removed dynamically, and you don't want nodes to continuously try reconnecting to a gateway that no longer exists, and won't exist.

For example:

1. Cluster A is created and is listening for gateway connections.
2. Cluster B is created, listening for gateway connections, and is configured to connect to cluster A via a gateway.
3. Then, cluster A is spun down and removed. All nodes in cluster B will continuously, as configured, try to reconnect to cluster A until they are restarted with a new configuration that doesn't have that gateway.

For my specific use case, I am trying to create a library that adds platform functionality to embedding NATS into Go projects. So for example if you deploy on Fly.io the library will automatically cluster nodes in the same region, and super cluster every region an app is deployed in. But, if in the example you remove "cluster A" (a specific region), the nodes in cluster B will try to connect ad infinitum.

Contribution

No response