Add support for new consumer attributes

[mmmries]

According to the NATS Consumer Docs there have been some new attributes added to consumers in nats-server 2.8.3 and 2.10.0.

We should support setting or checking these attributes, but we need to figure out some things like:

• If we attempt to send these attributes to earlier versions of the server, will it fail?
• Should we have different versions of the Consumer struct depending on which versions of the server we're connected to? Or just provide a default value in those new attributes when we lookup consumers from older clusters?