Allow providing http Agent in config.client.service

Currently the only things we can provide `client.service` in `apollo.config.js` are url, headers, and a skip SSL flag.

We have to build out quite a few work arounds to this limited configuration because of internal requirements of security. All of these workarounds could have been avoided if the configuration would allow providing an HTTP(S) agent directly from config, such as this here: https://github.com/apollographql/apollo-tooling/blob/bedd6eafbb8c1b690102738573069effd478283f/packages/apollo-language-server/src/providers/schema/endpoint.ts#L118

Something like the following would be ideal:
```js
if (config.fetchOptions) {
options.fetchOptions = config.fetchOptions;
} else {
// ... current logic
}
```
Though this would also work:
```js
if (config.agent) {
options.fetchOptions = { agent: config.agent };
} else {
// ... current logic
}
```

Start with the linked endpoint.ts example and trace how client.service is read from apollo.config.js and passed into the HTTP request options. Confirm the intended configuration shape and identify the relevant existing request tests; done means an HTTP(S) agent can be supplied through config.client.service without breaking the current URL, headers, or skip-SSL behavior.

Written by the indexing model from the issue text.