Cellular: Support for GPRS dial-up

[AriParkkila]

Description

Data state may be entered by issuing a 3GPP TS 27.007 command AT+CGDATA or a GPRS dial-up command ATD99**#.

This PR adds support for GPRS dial-up command on AT cellular network. This PR does not change any functionality until modem defines AT_CGDATA as unsupported feature.

Requested in Issue #7682 and PR #7304.

Pull request type

[ ] Fix
[x] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[AriParkkila]

@TeemuKultala please review

[cmonr]

@ARMmbed/mbed-os-test Another case of the smoke test logs being archived. Need a way to relaunch.

[0xc0170]

Restarting jenkins CI (should also restart smoke test).

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2899
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7870/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2524
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7870/

[mbed-ci]

Test : SUCCESS

Build number : 2650
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7870/2650