docstore: Support unwrapping action list errors

[Megakuul]

Example changes to use struct error types (see #3702 ).

While creating this PR I realized that there is already a concrete infrastructure for unwrapping driver specific errors (e.g. docstore.ErrorAs()). Besides, I also realized that rewriting this part is currently not very simple because it would inflict breaking changes in the driver<->interface API.

Therefore, I as initial step I added a new Wrap() function which simply wraps a gcerrs.Error into a code-specific struct (currently only implemented AlreadyExistsError).

The idea behind such an approach would be to slowly migrate to struct errors like this:

1. Slowly deprecate gcerror.ErrorAs() as it would now be possible to extract driver specific errors by simply using errors.As().
2. Instead of wrapping error codes, change the driver API to convert driver-errors to struct error wrappers (this step would break the API between driver<->sdk interface).
3. Remove legacy error helpers and deprecate error codes.

As already explained, I do now see that this is a long, complex and especially breaking process, therefore it is up to you to decide if the effort is worth going this way.

[vangent]

PTAL #3708 and see what you think. I think this solves the problem of being able to use errors.Is, taking advantage of the fact that Go CDK errors are already wrapped.

[vangent]

Please also update the PR title to reflect this is just about docstore's ActionList.