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

**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](https://github.com/danielgerlag/workflow-core/blob/12be88701780e1f1b2d5b5b5d98e7ce4df3e9913/src/providers/WorkflowCore.Persistence.EntityFramework/Models/PersistedSubscription.cs#L33C9-L33C9) 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.

Open src/providers/WorkflowCore.Persistence.EntityFramework/Models/PersistedSubscription.cs at the ExternalToken property and inspect its current MaxLength attribute. Verify how the EntityFramework provider persists this field and confirm that encoded tokens longer than 200 characters are accepted with the requested limit.

Written by the indexing model from the issue text.