Refactor elpa/oauth2.el to use plist over cl-defstruct

[Soham Gumaste]

Hello,

I have some more free time so I was looking into my oauth2.el again. I 
want to ask if it would be desirable to refactor the library to use 
plists in place of `cl-defstruct`? This would make more sense as the 
storage backend used is plstore, which uses plists as its native format. 
I have also read that the use of cl-lib is discouraged for various reasons.

My overall goal is to make oauth2.el integrate with auth-sources, and 
properly hook into functions like `auth-info-password` to refresh (if 
needed) and supply the token, in hopes to better integrate with nnimap 
in gnus.el.

Please let me know if you have any opinions on this. Please CC my email 
as I consume this list in digest form.

Thanks,
Soham
sohamg2@gmail.com