feat(mongodbflex): Refactor wait handler to use new WaiterHelper

[j1n-o9r]

Description

Update the wait handler of sfs, to use the new WaiterHelper struct. Replace every possible wait handler implementation of this service, to use just the WaiterHelper.

relates to STACKITSDK-382

Testing Instruction

• checkout feat/STACKITSDK-382-mongodbflex-refactor-wait-handler-to-use-wait-helper
• run make test
• go to your terraform repo and add this line to go mod, adjust path to sdk if necessary replace github.com/stackitcloud/stackit-sdk-go/services/mongodbflex => ../stackit-sdk-go/services/mongodbflex
• run tf acceptance tests for mongodbflex

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs
  • Changelog in the root directory was adjusted (see here)
  • Changelog(s) of the service(s) were adjusted (see e.g. here)
• VERSION file(s) of the service(s) were adjusted
• Code format was applied: make fmt
• Examples were added / adjusted (see examples/ directory)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)