feat: enable GitHub OAuth2 login by default on new deployments (#16662)

Third and final PR to address
https://github.com/coder/coder/issues/16230.

This PR enables GitHub OAuth2 login by default on new deployments.
Combined with https://github.com/coder/coder/pull/16629, this will allow
the first admin user to sign up with GitHub rather than email and
password.

We take care not to enable the default on deployments that would upgrade
to a Coder version with this change.

To disable the default provider an admin can set the
`CODER_OAUTH2_GITHUB_DEFAULT_PROVIDER` env variable to false.

coderd/database/queries/siteconfig.sql

@@ -107,3 +107,27 @@ ON CONFLICT (key) DO UPDATE SET value = $2 WHERE site_configs.key = $1;
 DELETE FROM site_configs
 WHERE site_configs.key = $1;
 
+-- name: GetOAuth2GithubDefaultEligible :one
+SELECT
+	CASE
+		WHEN value = 'true' THEN TRUE
+		ELSE FALSE
+	END
+FROM site_configs
+WHERE key = 'oauth2_github_default_eligible';
+
+-- name: UpsertOAuth2GithubDefaultEligible :exec
+INSERT INTO site_configs (key, value)
+VALUES (
+    'oauth2_github_default_eligible',
+    CASE
+        WHEN sqlc.arg(eligible)::bool THEN 'true'
+        ELSE 'false'
+    END
+)
+ON CONFLICT (key) DO UPDATE
+SET value = CASE
+    WHEN sqlc.arg(eligible)::bool THEN 'true'
+    ELSE 'false'
+END
+WHERE site_configs.key = 'oauth2_github_default_eligible';