Add bindings to backup & restore wallet

[tmalahie]

This PR adds the necessary bindings to perform the backup mechanism (rgblib_backup and rgblib_restore_backup)

[zoedberg]

Please see the requested changes.
After addressing those please squash the code from this PR into a single signed commit, named c-ffi bindings: add backup, restore_backup and backup_info. Finally rebase the commit on the updated master. Thanks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.29%. Comparing base (f04ff8d) to head (d2d1790).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #53   +/-   ##
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  Coverage   96.29%   96.29%           
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  Files          61       61           
  Lines       19966    19966           
=======================================
  Hits        19227    19227           
  Misses        739      739           

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[zoedberg]

This looks good, thank you!