* Moves CommandCompletionIT to Ledger API Test Tool
Closes#2841
Also refactors test observers into a single API and unifies the existing implementations of the DirectExecutionContext
* Allow explicit offsets to get checkpoints
* Raise checkpoint test timeout
* Consolidate checkpoint tests in a single test case
* Fix compilation issues
* Exclude command submission/completion tests for rev-v2
* Drop test not relevant for Ledger API implementations
* Fix compilation errors