Allow Users to set URL Path Prefix for Developer Playground

**Is your feature request related to a problem? Please describe.**
During local development we have many services running on a local k8s cluster. To prevent URL clashing, each service has its own URL prefix (e.g, `http://1.2.3.4/api/...`, `http://1.2.3.4/frontend-1/...`). I'm prefixing `cubejs` api calls with `http://1.2.3.4/cubejs/...` but the developer playground frontend has no ways to add a prefix to the api calls it makes.

**Describe the solution you'd like**
An environment variable like `CUBEJS_PLAYGROUND_URL_PREFIX` that can be set.

**Describe alternatives you've considered**
Temporarily allowing `http://1.2.3.4` to point to cubejs. It's rather cumbersome since I have to comment a bunch of code and then remember to uncomment before finishing a feature I'm working on.

**Additional context**