Add CI/CD Pipeline

[haraldhob]

Title

We propose the following CI/CD Pipeline for PGPro

Executions

• On PR to develop branch: Run tests and swift lint
• On push on develop branch: Build and sign app for Testflight
• On push on main branch: Build and sign app for App Store

To do for maintaining

Set the correct keys as GitHub Secrets:

• APP_STORE_CONNECT_API_KEY_CONTENT
• APP_STORE_CONNECT_API_KEY_ID
• APP_STORE_CONNECT_ISSUER_ID
• IOS_DIST_SIGNING_KEY (is base64 encoded)
• IOS_DIST_SIGNING_KEY_PASSWORD