[spec] add a ledger focused API spec

[amarts]

Define the API with 3 top level sections:

1. Ledger

   • Provide the interfacing for any ledger used as backend

2. Token Managers

   • Define the APIs used for token managers, which can be different at the parameter level, but always provides basic interfacing for any applications.
   • Consider different type of tokenized asset which would cover broad spectrum of usecases (for v1)

3. Application

• Any application specific APIs
• This may be covering more on User's (holders) Assets, and the possible actions which can be done on the asset.