feat: convert network_account and network_device_type to equinix-sdk-go

[ctreatma]

This PR is a companion to equinix/equinix-sdk-go#149 to exercise the Network Edge service code introduced there and cannot be merged until that functionality is released.

I have converted the network_account and network_device_type data sources to use equinix-sdk-go to demonstrate the process. Remaining data sources and resources can be converted gradually in follow-up PRs, especially since some of those may require additional features in equinix-sdk-go.

As part of the conversion process, I have also moved the affected data sources into the internal directory to align with #106.

[codecov]

Codecov Report

❌ Patch coverage is 5.63380% with 67 lines in your changes missing coverage. Please review.
✅ Project coverage is 27.94%. Comparing base (9094587) to head (7a7ed06).

Files with missing lines	Patch %	Lines
internal/config/config.go	0.00%	28 Missing ⚠️
...l/resources/network_edge/devicetype/data_source.go	4.00%	24 Missing ⚠️
...rnal/resources/network_edge/account/data_source.go	6.25%	15 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #959   +/-   ##
=======================================
  Coverage   27.93%   27.94%           
=======================================
  Files         239      239           
  Lines       31424    31460   +36     
=======================================
+ Hits         8779     8790   +11     
- Misses      22491    22516   +25     
  Partials      154      154           

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