Editors: allow optional service account linkage

[elenaviter]

Editors: allow optional default service account linkage if specified by FlowForge helm deployment.

Description

Editors: allow optional service account linkage if this specified by FlowForge helm deployment.
FlowForge helm relevant section is

editors:
  serviceAccount:
    create: true
    annotations: {}
    name: editors

Relevant environment variable is EDITOR_SERVICE_ACCOUNT.
*Note that by "default" here meant that all Editor instances will be provisioned with this service account linked.

Motivation: In general, there's a need and the plan for multiple IAM support, per Project, as a part of Project Configuration with the ability to change IAM binding (likewise the stack can be changed). To start simple, we consider a unified role linkage to unlock the cloud development on Editor instance, with that we link the default service account to Editor instances deployment via env variable.
Later this might be the definition "from project settings".

Related Issue(s)

• Editors: allow optional service account linkage
• FlowForge helm: 1. Editors: service account. 2. Broker: propagate ingress. 3. README

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on flowforge/helm to update ConfigMap Template
  • Issue/PR raised on flowforge/CloudProject to update values for Staging/Production

Labels

• Backport needed? -> add the backport label
• Includes a DB migration? -> add the area:migration label