feat: support the OAuth2 device flow with GitHub for signing in (#16585)

First PR in a series to address
https://github.com/coder/coder/issues/16230.

Introduces support for logging in via the [GitHub OAuth2 Device
Flow](https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/authorizing-oauth-apps#device-flow).

It's previously been possible to configure external auth with the device
flow, but it's not been possible to use it for logging in. This PR
builds on the existing support we had to extend it to sign ins.

When a user clicks "sign in with GitHub" when device auth is configured,
they are redirected to the new `/login/device` page, which makes the
flow possible from the client's side. The recording below shows the full
flow.


https://github.com/user-attachments/assets/90c06f1f-e42f-43e9-a128-462270c80fdd

I've also manually tested that it works for converting from
password-based auth to oauth.

Device auth can be enabled by a deployment's admin by setting the
`CODER_OAUTH2_GITHUB_DEVICE_FLOW` env variable or a corresponding config
setting.

coderd/coderd.go

@@ -1106,6 +1106,7 @@ func New(options *Options) *API {
 				r.Post("/validate-password", api.validateUserPassword)
 				r.Post("/otp/change-password", api.postChangePasswordWithOneTimePasscode)
 				r.Route("/oauth2", func(r chi.Router) {
+					r.Get("/github/device", api.userOAuth2GithubDevice)
 					r.Route("/github", func(r chi.Router) {
 						r.Use(
 							httpmw.ExtractOAuth2(options.GithubOAuth2Config, options.HTTPClient, nil),