Support multi-part and encoded SMS. Add endpoints to retrieve all SMS, retrieve/delete nth SMS, and retrieve network info

[gpailler]

Global improvements to support multi-part SMS with encoding

• Support multi-part SMS
• Support encoded SMS (should handle diacritics properly)
• Add new API endpoints:
  • retrieve all SMS
  • retrieve nth SMS
  • delete nth SMS
  • retrieve network info
• Update README.md:
  • document new API endpoints
  • update HomeAssistant section and add an automation sample to send a notification when a message is received

A docker image is available for testing gpailler/sms-gammu-gateway:multipart-sms