Extend /drep/list endpoint to apply search and filter functionality

[MSzalowski]

Why

To allow searching and filtering DReps.

What

extend /drep/list endpoint to include:

• filter - multi choice, based on DRep status (active, inactive, retired)
• sorting - based on date of registration, voting power, status (active > inactive > retired)
• Allow searching for DRepID.

How

Api should be simillar to GET /proposal/list for gov actions.
Describe in a comment final form that should be treated as acceptance criteria.

[pmbinapps]

DRep list is empty. Registration as DRep not possible due to problems with wallet.

[pmbinapps]

Checked here