feat: support user-supplied literal headers

[ethanndickson]

First step for #17.

This PR adds a settings menu to the app, and a network tab with a form for specifying hard-coded HTTP headers to be added on all outgoing requests. The settings cannot be modified while Coder VPN is on, as they can't be applied to the DERP headers without restarting the tailnet connection.

literalheaderflow.mov

Future work includes:

• Sending these headers to the Network Extension whenever they change, via XPC.
• Adding support for dynamic, shell command-based headers, as also supported by the CLI.

[ethanndickson]

• feat: support user-supplied literal headers #24 👈 (View in Graphite)
• chore: improve login form styling #23
• chore: extract CoderSDK to framework #19
• fix: set pipefail during make test #22
• main

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[ethanndickson]

I was debugging a CI-only test failure and xcpretty was swallowing a helpful error log - xcbeautify seems like it provides better output in general for tests.

For context, the default xcodebuild output is thousands of lines, and not at all readable.

[ethanndickson]

We need to discuss what minimum macOS version we want to target. On one hand, an app like Tailscale supports versions as old as 11 from 2020, but on the other hand, I suspect most enterprises will be keeping their company devices on 14 or 15, though it'd be nice to know for sure.

Fortunately, we could set this value to 13 right now, which is the earliest version Apple still 'supports' (they don't really have a policy FWICT, but the pattern seems to be security updates for the last three versions)

The big feature in 14 SwiftData, is a Swift interface for a sqlite-backed app database. I messed around with it for this feature, but it's definitely overkill here. It might be super useful in the future?