Note that the returned hook events are changed to reflect the actual parameter
names used to create or update the webhook. Specifically, the original
`convertEvents` in the GitLab driver sets the value of `Hook.Events` with the
following mapping:
```
issues_events=true -> "issue"
tag_push_events=true -> "tag"
push_events=true -> "push"
note_events=true -> "comment"
merge_requests_events=true -> "merge"
other options -> none
```
It seems the event name strings are arbitrary and not consistent across
different drivers, so the mapping itself doesn't provide much abstraction (if
any). More importantly, for all other drivers, `Hook.Events` contains native
events unrecognized by go-scm, but GitLab driver completely discards them. This
is important to implement logics to "reconcile" webhooks (i.e., update only if
needed) to sync the list of watched events. Therefore, this patch proposes
`Hook.Events` returned by the hook-related functions in the GitLab driver to
store the option names if their values are true, e.g., `["issues_events",
"push_events", "confidential_issues_events"]`.

This is a breaking change.