[NT-88] Send client_id in request headers

[ifbarrera]

📲 What

With requests going to the v1 API, we would usually send the client_id as a query param. For /graph requests (which are all POST requests) we'd rather send the client_id in a custom header. For consistency, this PR adds the new custom header to all our requests.

🤔 Why

Increased security for /graph requests.

🛠 How

Added X-KICKSTARTER-CLIENT to our defaultHeaders.

✅ Acceptance criteria

This can be tested using Charles proxy.

• Verify that x-kickstarter-client header appears in /v1 requests (ex. discovery screen)
• Verify that x-kickstarter-client header appears in /graph query requests (ex. viewing stored cards)
• Verify that x-kickstarter-client header appears in /graph mutation requests (ex. adding a new stored card)

[justinswart]

Nice thanks!