feat(platform): support custom registries for managed providers

[jezekra1]

Create credentials secret, then use it in helm values, locally:

# 1. start cluster
beeai platform start

# 2. create secret
beeai platform exec -- kubectl create secret docker-registry regcred ...

# 3. update platform
beeai platform start --set 'imagePullSecrets[0].name=regcred'