Add link to APIKeyMiddleware library

[ptrkstr]

• Project Name: APIKeyMiddleware
• Project URL: https://github.com/ptrkstr/APIKeyMiddleware
• Project Description: Swift package for adding API Key requirement to vapor backends.
• Why it should be added to awesome-vapor: Simplifies adding additional security to vapor backends.
• Project has at least 15 stars (if it’s a GitHub project) - 🥺
• Project supports at least Vapor 3 - supports Vapor 4
• Project supports at least Swift 5
• I used correctly sized dash and ended the entry with a full stop 🤓