Refactor pemfile.Options as the intermediate struct is not needed

[gtcooke94]

The only reason for using the intermediate struct is to be able to specify a default value for the refresh interval, this could be done with the encoding/json package as follows: https://stackoverflow.com/questions/30445479/how-to-specify-default-values-when-parsing-json-in-go

See https://github.com/grpc/grpc-go/pull/8180/files#r2002089915

[hugehoo]

Hi, do you mind if i work on this issue?

[injae-kim]

Fix PR by @hugehoo : #8334

[arjan-bal]

During the review we decided that the fixes didn't offer a significant improvement over the existing implementation, so we're not making any changes.
#8334 (comment)