add imageregistry prefix to timescaledb images

[gugu]

Right now timescaledb does not support container proxies, so I've added support for common .global.imageRegistry chart parameter to remove dependency on dockerhub

[gugu]

@itay-grudev, feel free to contact me if you have any questions/requests for this PR

[itay-grudev]

@gugu If we are adding this can you add it everywhere? Otherwise it is a bit confusing what it applies to.

Also, would you please add a test for this in: https://github.com/cloudnative-pg/charts/tree/main/charts/cluster/test/postgresql-cluster-configuration

This tests whether non-default configuration properties get forwarded correctly and ensures we don't accidentally break something in the future.