Support using a custom service account

[nkeenan38]

Description of the change

Adds an option (backend.serviceAccount.name) to use a non-default service account for the statefulset. The option backend.serviceAccount.create allows the service account to be managed by the helm chart or externally.

Existing functionality is unchanged because the default values continue to use the default service account.

Type of change

• [ ] Bug fix (non-breaking change that fixes an issue)
• [x] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issues

Checklists

Development

• [ ] Lint rules pass locally
• [ ] The code changed/added as part of this pull request has been covered with tests
• [ ] All tests related to the changed code pass in development

Code review

• [ ] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [ ] "Ready for review" label attached to the PR and reviewers mentioned in a comment
• [ ] Changes have been reviewed by at least one other engineer
• [ ] Issue from task tracker has a link to this pull request

[bdashrad]

Didn't see this when I opened it, but this was closed by #65