Use existing serviceAccount within Helm deployment

[ruzmuh]

Is your feature request related to a problem? Please describe.
Currently, the Helm chart doesn't allow the use of an existing serviceAccount because a new serviceAccount is unconditionally created during the Helm deployment.

Describe the solution you'd like
We should add a serviceAccount.create boolean variable to give users the option not to create a serviceAccount.

Describe alternatives you've considered
None.

Additional context
This feature would be helpful in situations where serviceAccounts are created separately from Helm charts.