Add CORS whitelist as env variable

[JensWinkler91]

Is your feature request related to a problem? Please describe.

I've found this old issue about cors #950.
The suggested solution there is to use a proxy server to access the api via browser.

In my case that would mean that i have to implement a new backend instance or configure a proxy server that has the only purpose to forward my request from localhost:3000 to localhost:8080/v1/update.

As watchtower provide an API i think there should be a possibility to define a CORS whitelist.

Describe the solution you'd like

I'd be nice if we could use a variable .e.g. WATCHTOWER_HTTP_CORS_ORIGINS to allow one or multiple origins.

As env variable:
WATCHTOWER_HTTP_CORS_ORIGINS=localhost:3000, localhost:4000

Describe alternatives you've considered

I don't know other alternatives.

Additional context

No response