Add list assigned form templates for NGO Admins

[idormenco]

Features:

• add a new endpoint in project Feature.FormTemplates GET /api/form-templates/election-rounds/{electionRoundId}:available
• this route is for NGO admins olny
• returns a paginated list of assigned form templates.
• If there are no form temlates assigned return empty response
• add unit & integration testing

note
see GET /api/election-rounds/{electionRoundId}/forms

[FriptuT]

I'll take this

[idormenco]

@FriptuT
all yours