Set Accept header to 'application/json' for GraphQL queries

[pkukielka]

Changes

This PR fixes setting Accept header to 'application/json' which is required by some proxies to work.

Test plan

Tested with a local reverse proxy.

Prerequisites (YOUR_TOKEN is a token configured for your user at YOUR_INSTANCE):

1. Right cody_setting.json configuration:

{
  "cody.override.serverEndpoint":  "https://localhost:443",

  "cody.auth.externalProviders": [
    {
      "endpoint": "https://localhost:443",
      "executable": {
        "commandLine": ["echo '{ \"headers\": { \"Authorization\": \"token YOUR_TOKEN\" } }'"],
        "shell": "/bin/bash"
      }
    },

2. Proxy running in the console: mitmproxy -m reverse:https://YOUR_INSTANCE/ -p 443

Then you can start cody, and in the proxy console you should be able to inspect one of the graphql requests and see something like this: