fix!(helm): adjust the logic for serviceAccount creation

[burningalchemist]

• serviceAccount field is deprecated in favour of serviceAccountName;
• serviceAccount.create should create a serviceAccount resource bound to the helm release name (which can be overriden, yet is consistent);
• serviceAccount.name should allow users to reference a custom service account created outside of the helm release;
• if create is false, and name is not set, serviceAccountName field falls back to default which is equal to not providing the serviceAccountName at all.

BREAKING CHANGE: from v0.5.0 serviceAccount sql-exporter is created by default. Can be disabled if needed to reference a custom serviceAccount object.