Designation event extension

[roman-khimov]

Is your feature request related to a problem? Please describe.

I'm always frustrated when we don't have protocol parity with C# node. neo-project/neo#3397 has implemented an event extension.

Describe the solution you'd like

Port it to NeoGo.

[AnnaShaleva]

We can implement it, but just a note: this change was not merged into neo-project master.

[roman-khimov]

I know, but it's a technical detail mostly now. Looks like we'll have some gigantic hf_echidna branch (like a dev branch) that will then be merged without any additional changes.

[AnnaShaleva]

Depends on #3554.