Create new `NethLink` extension in Wizard UI/API

[edospadoni]

To configure NethLink it is necessary to expose this configuration in the Wizard UI and create a related extension for the extension configuration.

PRs released on Beta2:

• https://github.com/nethesis/nethvoice-wizard-ui/pull/192
• https://github.com/nethesis/nethvoice-wizard-restapi/pull/229
• https://github.com/nethesis/nethcti-server/pull/307
• https://github.com/nethesis/nethcti3freepbx/pull/52

PRs released on RC1:

• https://github.com/nethesis/nethcti-server/pull/309
• https://github.com/nethesis/ns8-nethvoice/pull/170