feat: allow usage of existing secret for preshared keys

[bogirstc]

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]

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