at_c: use atclient_connection_read in atclient_get_atkeys

[JeremyTubongbanua]

Currently, we have a fixed read buffer of 4096 bytes. If scan outputs a larger output of 4096 bytes, then not all keys will be captured in the read.

Therefore, we should use the new atclient_connection_read implementation where a maximum buffer size can be specified through atclient_get_atkeys_request_options

[JeremyTubongbanua]

Making this P2 because atclient_get_atkeys currently won't work for large scans