Add NACM support

[pepa-cz]

Hello,

I would like to request support for NACM (NETCONF Access Control Model) in sysrepo-python. This would allow applications to evaluate access permissions (to configuration, RPCs and more) directly via Python, using the NACM configuration already present in sysrepo.

I have already implemented this and opened pull request #84.

[pepa-cz]

Hello, just checking in to see if there's any update on this PR. Please let me know if anything needs to be changed or clarified.

[pepa-cz]

Merged.