oktaAuth = async (req, res, next) => {
    const authHeader = req.headers.authorization || '';
    const match = authHeader.match(/Bearer (.+)/);
  
    if (!match) {
        res.status(401);
        return next('Unauthorized');
    }
  
    const accessToken = match[1];
    try {
        const jwt = await oktaJwtVerifier.verifyAccessToken(
                accessToken, 'api://default');
        req.jwt = jwt;
        return next();
    } catch (err) {
        functions.logger.log('Unable to verify Okta access token', err);
        res.status(401);
        return next('Unauthorized');
    }
}
Implements the `/api/firebaseCustomToken` endpoint, which is responsible for
validating Okta access tokens and minting Firebase custom authentication
tokens.