feature(MCD): Add MCD support

[aks96]

Description

This PR implements Multiple Custom Domains (MCD) support, enabling application to dynamically authenticate users against multiple Auth0 tenants or custom domains based on request context.

Changes

Dynamic Issuer Resolution: issuerBaseURL now accepts a function (context: { req }) => string | Promise in addition to a static string
Per-Issuer Metadata Caching: New IssuerManager class caches OIDC discovery metadata per issuer URL
Session-Bound Issuers: Sessions store session.issuer to ensure token refresh and logout use the correct issuer

Usage Examples

Dynamic Resolver

const { auth } = require('express-openid-connect');

const tenantConfig = {
  'tenant-a': 'https://tenant-a.us.auth0.com',
  'tenant-b': 'https://tenant-b.eu.auth0.com',
  'default': 'https://default.auth0.com',
};

app.use(auth({
  issuerBaseURL: async (context) => {
    const tenantId = context.req.query.tenant || 'default';
    return tenantConfig[tenantId];
  },
  authRequired: false,
}));

Test

Added sample test multiple-custom-domain.js.