[dashboard] make multiple service endpoints configurable

[coesensbert]

One can set multiple grid backend service endpoints on a dashboard deployment, in case one goes down the dashboard has another to query. This list will have to be set on a custom basis for each individual deployment.

use the .env to define custom endpoints with a pre defined count on each network (seen the current stack deployments). Make clear notes in the docs and as comments how this should be used.

• devnet
• deploy and test on devnet
• qanet
• testnet
• mainnet

Repo: https://github.com/threefoldtech/tfgrid-sdk-ts/blob/development/packages/playground/docs/build.md#required-env-vars
See: https://github.com/threefoldtech/tf_operations/issues/3074