DSL support for accounts

[abhijeetkaurav1st]

Added account models for various provider type. Added sample examples under tests/accounts directory.

Doc: https://docs.google.com/document/d/1qABJgsQSXkc7kw6e2n7GJc2fRCHrDJyAspI3SWbwmt0/edit

Commands added:

1. calm compile account -f <file_name> : Compiles the account.
2. calm create account -f <file_name>: Create an account.
3. calm verify account <account_name>: Verifies the account.