feat: configurable timeouts for environment instance and service instance

[lechnerc77]

Purpose

• Introduce configurable timeouts for the resources subaccount_environment_instance and subaccount_service_instance
• Record new test cases
• Enhance documentation
• Fixes [FEATURE] Configurable timeout for resources #11

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 ./...

What to Check

Verify that the following are valid:

• Automated tests are executed successfully

Other Information

We restrict the current configurability to the resources subaccount_environment_instance and subaccount_service_instance as they have the largest range of time needed for the create, update and delete operations. In case of customer feedback we can transfer it to further resources
This avoids unnecessary duplication of the datatypes for the data sources.

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.

[v0lkc]

Just a minor finding with the formatting, otherwise good!