[FEATURE] Move of login logic

[lechnerc77]

In the current version of the Terraform provider (as of today 0.3.0) The login is part of the provider.go.
This spot has the disadvantage that the logon to BTP is triggered quite often due to the lifecycle of Terraform (see test fixtures) and makes the usage of the RSA token in case of active 2FA a hard task.

The intention of this feature or enhancement would be to move the login flow to a place later in the life cycle ("lazy login") to reduce the amount of logons and make the usage of a 2FA token possible as it is the case for the BTP CLI or the Cloud Foundry provider.

[lechnerc77]

Effort-wise it must be considered that all test fixtures must be re-recorded