feat: add Hunter plugin

Summary

Adds Hunter plugin with the following action:

• Enrich Lead: Get data on a lead and their company using Hunter's enrichment APIs

Accepts an email address with three enrichment types:

• Individual: Person data only via /v2/people/find
• Company: Organization data only via /v2/companies/find
• Combined: Both person and company data via /v2/combined/find

Test plan

• Connection test validates API key via /v2/account endpoint
• Enrich Lead action executes successfully with valid email
• Invalid email format returns appropriate error
• Individual enrichment returns person data only
• Company enrichment returns organization data only
• Combined enrichment returns both person and company data
• Error handling works for invalid API keys and emails not found