Add TokenStore capabilities

**Is your feature request related to a problem? Please describe.**
I'm trying to get custom authentication to work, I've managed to get the login to work, I've got an x-ms-principal but I need to get refresh and access tokens from this authentication so that I can call other services in our backend.

Right now there seems to be support for this in appservice, https://docs.microsoft.com/en-us/azure/app-service/configure-authentication-provider-openid-connect.

I cannot find anywhere that it is possible to enable this tokenstore for static web apps?

**Describe the solution you'd like**
Support the token store setting like app-service does.

**Additional context**
I might have missed some configuration, but I think this feature is not available right now?