electron: use standalone DNS client for all modem types

[avtolstoy]

Problem

We've seen SARA U2XX modems stuck returning potentially cached DNS lookup results for longer than records' TTL despite the fact that DNS records have been modified. The only thing that caused AT+UDNSRN to correctly re-resolve in those particular cases was to reset the modem.

This causes problems for device service deploys.

Solution

In order to remove this layer of uncertainty with AT+UDNSRN we are using our own DNS client on all Gen 2 cellular devices now, whereas previously we've only used it for LTE SARA R4 devices where AT+UDNSRN was not supported.

Steps to Test

N/A

Example App

N/A

References

• [CH39529]

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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)

[sergeuz]

The changes look good 👍 The only suggestion I have is to use the network's DNS address instead of 8.8.8.8 on platforms where we can retrieve that address. It seems unlikely that the network's DNS would treat TTL incorrectly as well (we'll need to test that of course)