validate rfc8040 content parameter

there is already some work in robot code that uses the content parameter so we can
build on that. Essentially, there are 3 options plus the option without the content
parameter. content=config, content=nonconfig and content=all

config is similar to our bierman calls for the config store. nonconfig is same as operational
and all should be a combination of both (afaik). not having a content filter should be
the same as all.

I don't know if this has been looked in to very deeply, but doing that and making sure we
have some CSIT to cover it is what this ticket is about.

rfc details