Improving assets/balances data flow on nim side

[Khushboo-dev-cpp]

Description

In the discussions here https://www.notion.so/Working-group-for-nim-layer-improvements-cb67f4b26b4242d9a31ada4af5959e89 the improvements done to the nim layer (https://miro.com/app/board/uXjVKghE6b8=/) was discussed and while everyone agreed this to be step in the right directions.

Goals:
1.Simplify flow further and make it more clean and streamlined.
2. Find a way to pass events from service via the interface itself so that the flow can be easily replicated in other parts of the app.
3. Submodel handling. (can we find a better way to handle those)

Created this GH issue so that its a workable task with a concrete result from it.
@micieslak @saledjenic @alexjba please add more goals we'd like to achieve here in case Iv missed something.