feat: allow usage of existing secret for preshared keys

[bgdn-r]

Summary

Allow usage of existing secret that contains the pre-shared keys.

Description

Extending the Helm chart with the option to read the pre-shared keys from a Kubernetes secret.
The secret is expected to have the presharedKeys key which will contain the keys themselves.
The functionality is the same as for datastore.uriSecret.

References

#175
#188

[linux-foundation-easycla]

• ❌ login: @scbogdan / The commit (470f704). This user is authorized, but they must confirm their affiliation with their company. Start the authorization process by clicking here, click "Corporate", select the appropriate company from the list, then confirm your affiliation on the page that appears. For further assistance with EasyCLA, please submit a support request ticket.