Add support for Prestop hook to execute custom commands

[prasoon-pandya]

Is your feature request related to a problem? Please describe.
Currently there is no way to provide custom commands that will run during pre stop hook, there is an option to specify such commands for post start hook. In some cases users may want to run commands like leader step down for graceful Vault restart, which is not currently possible.

Describe the solution you'd like
A helm chart configuration variable to specify custom pre stop commands to be executed.

Describe alternatives you've considered
There is no alternative to this feature.

Additional context
Link to helm chart template where configuration option need to be added:

vault-helm/templates/server-statefulset.yaml

Line 208 in b3daaad

preStop: