Increase the [MaxLength] for ExternalToken in PersistedSubscription in EntityFramework provider

[martikyan]

Is your feature request related to a problem? Please describe. If activities have encoded tokens longer than the current limit of 200 characters, the ExternalToken will not be persisted.

Describe the solution you'd like I would like to suggest increasing the [MaxLength] attribute for ExternalToken to a more reasonable length. 500 seems to work fine.