WIP: Implementation of setting initial EPS bearer for ublox

The intent of this merge request is to implement setting of the initial EPS bearer in u-blox modems based on guidance given in a document provided by u-blox, which can be found here:

https://www.u-blox.com/sites/default/files/LTE-initial-default-bearer_AppNote_%28UBX-20015573%29.pdf

As outlined in the document, there are different methods to be followed for different models of modems, and it is assumed that for the models not mentioned in the document that the setting of the initial EPS bearer is non-applicable.

In the WIP code, I have a basic structure for how the code might work (based on the implementation of a similar feature in the Cinterion plugin) as well as some comments for sections of it that still need to be fleshed out. Because the methodology varies based on model, I was thinking that the predefined per-model band configuration array given in mm-modem-helpers-ublox.c could potentially be expanded and generalized to list the proper initial bearer method (or none) for each model as well. Is this an acceptable approach? Or would there be a better way to handle this?

Another question I have -- with this implementation, I don't believe the steps for loading the initial EPS bearer are required, as the CID will always be the same and the APN and IP configuration will eventually be provided by NetworkManager when I implement it calling the SetInitialEPSBearerSettings() method. Is my understanding correct in this, that we will be able to depend on NetworkManager to provide the necessary information and just write it into the modem when triggered, rather than needing to load it through other means?

In general, I am looking for some advice on this implementation and to see whether or not I am on the right track -- any help and guidance that could be provided would be greatly appreciated. @aleksm -- I hope it's not rude to ping you, but do you have any thoughts on this?