CORS support for Apollo Client Awareness headers

[sjungling]

Is your feature request related to a problem? Please describe.
While testing router, I ran into issues with my app and disallowed headers.

Eventually, I traced it down to the name option on the ApolloClient constructor which is a part of the Client Awareness functionality that sets apollographql-client-name header on all requests.

Once I added that header to the allowed headers CORS configuration, it worked like a charm.

Describe the solution you'd like
It would be nice if these two headers were a part of the default Allow Headers for the CORS configuration.

Describe alternatives you've considered
An alternative is to have folks define those allowed headers in the router config. This could be accomplished, in part, with documentation about this.

Additional context
Add any other context or screenshots about the feature request here.

[abernix]

Thanks for opening this! I imagine this could be another configuration "knob" that could be in a similar place as where the config for #373 ends up. I've tacked this on as a "todo" in #66 for consideration, too.

[abernix]

I think an approach here could be:

• Set the Apollo headers as the default CORS headers
• Document that if you don't want to use the defaults, you need to put some CORS configuration in place.