Add CORS setup

[wtrocki]

It seems that docker image provided is not working. Some issues we saw when gw is containerized.

• Cors are not enabled, so impossible to hook in OVP queries into it.

• I need to change the network interface to 0.0.0.0 instead of localhost, to get me to work expose the 8080 and use GraphiQL IDE it from my browser.

• Somehow I cannot proxy requests deployed. This happens also in my machine locally as I am seeing these warning messages when starting gw:
  docker run -it --rm -p 4000:8080 quay.io/graphql/ovp-gateway
  2020/10/13 15:34:18 loaded previously stored schema: upstreams/gateway.graphql
  2020/10/13 15:34:18 loaded previously stored schema: upstreams/serverless1.graphql
  2020/10/13 15:34:18 The upstream does not have any query entry points
  2020/10/13 15:34:18 The upstream does not have any query entry points
  2020/10/13 15:34:18 The upstream does not have any mutation entry points
  2020/10/13 15:34:18 The upstream does not have any subscription entry points
  2020/10/13 15:34:18 GraphQL endpoint is running at http://[::]:8080/graphql
  2020/10/13 15:34:18 Gateway GraphQL IDE is running at http://[::]:8080/graphiql

[wtrocki]

Could not figure out how to add cors stuff easily.

[lmnzx]

Since this project uses the "net/http", I think we have to use an external library to implement cors, https://github.com/rs/cors seems to a great library for this purpose.

[wtrocki]

@snxk Amazing choice!

@chirino let us know if you are against it. If not we going to add that

[wtrocki]

Anyone could work on this. Let's do quick fix first - setup hardcoded header - which is one liner. Then we can extract config

[chirino]

Sounds good to me.