Automation of xsuaa credential management

[JohannesRu]

Problem Statement

While it already is possible today to orchestrate your role collections and assignments using the provider, usage up to this point, still requires you to manually create and inject API credentials for the xsuaa API. This kind of breaks the whole idea of full automation since the credentials can only be created once the subaccount is ready, which is usually part of the orchestration itself as well.

Idea

We can now automate this step as well with upjetting a new terraform resource to manage the lifecycle of this credentials:
https://registry.terraform.io/providers/SAP/btp/latest/docs/resources/subaccount_api_credential.

This will make basically be the same concept as we commonly use, e.g. for the connection between CloudManagement and CloudFoundryEnvironment.

Additional Links:

We teased the concept in the community pages already:
https://pages.github.tools.sap/cloud-orchestration/docs/sap-services/btp-services/account-managment/usermanagement