[Feature]: Support .netrc based authentication in the ASF module for earthdata

[whatnick]

Is your feature request related to a problem? Please describe.
I manage my credentials for earthdata via earthaccess using .netrc file. It would be nice to reuse this with PyGMTSAR for SLC download as well.

Describe the solution you'd like
PyGMTSAR wraps earthaccess or other more generic data access protocol as federated SAR data access expands beyond ASF and provides credential management options.

Describe alternatives you've considered
Hardcoding credentials in saved notebooks is a security risk, can create a custom .netrc reading function to use with PyGMTSAR.

[AlexeyPechnikov]

Sure, you can read and parse the .netrc file to get the credentials. But Earthdata servers are public and must be accessible to everyone, and the password requirement is just a fake because there are no user reviews or activity checks. I’ve inserted the credentials in the notebooks to allow them to be running in a single click, and as I know NASA and ESA staff use them in this way too :) Notably, the single user record can serve hundreds or thousands of users daily and it proves there are no activity checks or restrictions on the server side. Alternatively, we could create a new temp account for every run but of course these stored records can overload NASA servers and we will spend a century to fix the issue and restore access.