Add /account service for the usage of Proposal Discussion Forum pillar

[MSzalowski]

Add new endpoint:

/account/${stakeKey}:

{
  "id": number,
  "isRegistered": boolean,
  "view": string,
  "isScriptBased": boolean
}

On the backend side:

1. Add new service (backend/service/Account.hs)
2. Create new sql (depend on stake_address table)
3. Create new endpoint in API.hs /account/{stakeKey}
4. Create swagger documentation

On the frontend side:

1. Write query logic to GET /account/${stakeKey}
2. Pass query to Proposal Discussion Forum: frontend/src/pages/ProposalDiscussion.tsx

For the sake of testing results can be compared with Koios