Make persistent secret generation for `adminSecret`

[colearendt]

Is your feature request related to a problem? Please describe.

This is related to #7 , although that issue seems a bit dated. Basically, when createAdminSecret is used (by default), a new secret seems that it will be generated on each release (I have yet to verify myself).

Describe the solution you'd like In line with #73 , it is possible to generate values in such a way that they can be used consistently and preserved within Kubernetes (along with an optional user notification).

Describe alternatives you've considered Putting the work onto the user to maintain / change these values (as is currently done)

Additional context

Depending on the outcome of #73, this would be fairly straightforward to implement within the helm chart. Wording and whether/how to notify in NOTES.txt (i.e. once for each value? Something less verbose?) will be important to consider.