Add exporter for Asterisk RealTime SIP/Extensions

[michael-batz]

A new exporter should be added to export fields of CMDB objects to a Asterisk RealTime SIP/Extensions database.

ToDos:

• [x] create exporter
• [x] tests
• [x] documentation

[michael-batz]

Initial version implemented. Implementation ToDo:

• handling for missing/misconfigurated phone numbers in extensions
• make SIP context and host configurable
• error handling

[michael-batz]

Implementation und documentation done. Now testing.

[michael-batz]

tests completed