feat: add Beehiiv plugin

Summary

Adds Beehiiv plugin with the following actions:

• Create Subscription: Create a new subscription for a publication
• Get Subscription: Get subscription details by email
• Update Subscription: Update subscription details by email
• Add Subscription Tag: Add tag(s) to a subscription
• Add to Automation: Add an existing subscription to an automation flow

Test plan

• Connection test validates API key and publication ID
• Actions execute successfully in a workflow
• Error handling works for invalid inputs