[Gen2] Add cellular registration intervention commands to Gen2

[keeramis]

Problem

Add cellular AT intervention commands to Gen2, to improve/speed up registration process. This is particularly helpful in scenarios where the radio struggles to connect to an invalid operator.

Solution

1. Added CellularRegistrationStatusMdm similar to the one in Gen3 to run intervention commands
2. Replaced the csd, psd, and eps members of NetStatus with CellularRegistrationStatusMdm's status methods for consistency.

Steps to Test

Run any tinker-like app on Gen3 which enables connecting to the network. Try removing the antenna or using an invalid SIM, even better if there are known network issues in a given area to check how the the intervention commands run and help register to a network.

Example App

void setup() {
}

void loop() {
}

References

• ch69276

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

Looks good, rejecting only to remove code duplication with CellularRegistrationStatusMdm.

Double check AT+CGATT vs AT+CGACT.