API for retrieving assistance data validity

Quectel modems have an AT command for querying the validity of the XTRA assistance data. It would be nice to have this as a property in the ModemManager D-bus API as well, so that the data can be downloaded and injected again when it is becoming invalid.

Here's the output on EG25 (Pinephone) after injecting the correct assistance data (from http://xtrapath3.izatcloud.net/xtra3gr.bin)

$ mmcli -m any --location-inject-assistance-data=xtrapath3-xtra3gr.bin
successfully injected assistance data
$ mmcli -m any --command='AT+QGPSXTRADATA?'
response: '+QGPSXTRADATA: 10080,"2021/09/19,15:00:00"'

The first value is the number of minutes the data is valid (10080 = 1 week) and the date string gives the start of the validity.

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