Feature/update get token for custom api urls

[thinkjrs]

This adds promoApiUrl to the getToken function signature so that users can add a root api url for customizability.

For example, users can now call the function as such.

const refreshToken = getToken(accessToken, 'https://feature.promo.api.tincre.dev');

[github-actions]

size-limit report 📦

Path	Size
dist/promo-button-node.cjs.production.min.js	57.14 KB (+0.02% 🔺)
dist/promo-button-node.esm.js	57.23 KB (+0.05% 🔺)