feat: handling of "not found" resources in READ operations

[lechnerc77]

Purpose

• Adopting the best practices for handling of READ operations in resources we adjust the error handling as already available for subaccount trust configurations (see PR feat: remove state in case of 404 errors to enable recreation #513) for the other resources of the provider that have a READ operation implemented via API calls
• The test setup that landed in the provider via PR feat: remove state in case of 404 errors to enable recreation #513 was removed as the fixture was created manually and cannot be automatically recorded due to a gap in the testing framework that might be closed in the near future (for details see Add RemoveState option to TestStep hashicorp/terraform-plugin-testing#118 (comment))
• Resources that do not have a READ operation via an API call were omitted
• Follow-up issue for testing is created: [FEATURE] Improve testing for handling of manually deleted resources #519
• Closes [FEATURE] Handling of removed resources in READ #514

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[X] Feature
[ ] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

How to Test

• Test the code via automated test

go test ./...

• Additional test steps

- Create resources via Terraform
- Manually remove the resources in the BTP cockpit
- Re-executing a terraform plan

What to Check

Verify that the following are valid:

• Automated tests are executed successfully
• Missing resources trigger a recreation without manual intervention by the caller

Other Information

• For best practices around READ see https://developer.hashicorp.com/terraform/plugin/framework/resources/read
• For missing functionality in testing framework see Add RemoveState option to TestStep hashicorp/terraform-plugin-testing#118 (comment)

Checklist for reviewer

The following organizational tasks must be completed before merging this PR:

• The PR is assigned to the Terraform project and a status is set (typically "in review").
• The PR has the matching labels assigned to it.
• The PR has a milestone assigned to it.
• If the PR closes an issue, the issue is referenced.
• Possible follow-up items are created and linked.

[lechnerc77]

Sonar Cloud fails due to aforementioned limitation concerning the testability. Overall test coverage will not be decreased, so the failed check can be ignored