Refactor Backend APIs to follow REST standard

### Why

The Swagger interface for API is available here:
https://govtool.cardanoapi.io/swagger-ui/

The api follow non-standard naming and the paths contain redundant words like 'list' 'get' etc.

E.g. This is how current DRep APIs look like

```
GET /drep/list -> List all DReps in the network
GET /drep/info/{drepId} -> Get detail about particular DRep
GET /drep/get-voting-power/{drepId} -> Get voting power of a DRep
GET /drep/getVotes/{drepId} -> Get votes casted by a DRep
```

### What

Refactor the api to follow proper naming conventions.
Guideline : https://restfulapi.net/resource-naming/

### How

More standard and consistent way to represent those resource should be used.

Example for DRep resources.
```
GET /dreps -> List all DReps in the network
GET /dreps/{drepId} -> Get detail about particular DRep
GET /dreps/{drepId}/voting-power -> Get voting power of a DRep
GET /dreps/{drepId}/votes -> Get votes casted by a DRep
```