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.sql.go

@@ -8100,6 +8100,23 @@ func (q *sqlQuerier) GetNotificationsSettings(ctx context.Context) (string, erro
 	return notifications_settings, err
 }
 
+const getOAuth2GithubDefaultEligible = `-- name: GetOAuth2GithubDefaultEligible :one
+SELECT
+	CASE
+		WHEN value = 'true' THEN TRUE
+		ELSE FALSE
+	END
+FROM site_configs
+WHERE key = 'oauth2_github_default_eligible'
+`
+
+func (q *sqlQuerier) GetOAuth2GithubDefaultEligible(ctx context.Context) (bool, error) {
+	row := q.db.QueryRowContext(ctx, getOAuth2GithubDefaultEligible)
+	var column_1 bool
+	err := row.Scan(&column_1)
+	return column_1, err
+}
+
 const getOAuthSigningKey = `-- name: GetOAuthSigningKey :one
 SELECT value FROM site_configs WHERE key = 'oauth_signing_key'
 `
@@ -8243,6 +8260,28 @@ func (q *sqlQuerier) UpsertNotificationsSettings(ctx context.Context, value stri
 	return err
 }
 
+const upsertOAuth2GithubDefaultEligible = `-- name: UpsertOAuth2GithubDefaultEligible :exec
+INSERT INTO site_configs (key, value)
+VALUES (
+    'oauth2_github_default_eligible',
+    CASE
+        WHEN $1::bool THEN 'true'
+        ELSE 'false'
+    END
+)
+ON CONFLICT (key) DO UPDATE
+SET value = CASE
+    WHEN $1::bool THEN 'true'
+    ELSE 'false'
+END
+WHERE site_configs.key = 'oauth2_github_default_eligible'
+`
+
+func (q *sqlQuerier) UpsertOAuth2GithubDefaultEligible(ctx context.Context, eligible bool) error {
+	_, err := q.db.ExecContext(ctx, upsertOAuth2GithubDefaultEligible, eligible)
+	return err
+}
+
 const upsertOAuthSigningKey = `-- name: UpsertOAuthSigningKey :exec
 INSERT INTO site_configs (key, value) VALUES ('oauth_signing_key', $1)
 ON CONFLICT (key) DO UPDATE set value = $1 WHERE site_configs.key = 'oauth_signing_key'