Add pagination helper to easily collect all items

[timofurrer]

🚧 THIS IS MERELY AN EXPERIMENT FOR NOW! 🚧

The #1496 Pull Request triggered
some thinking for how to do this in a more "generic" way.

Thus, I drafted this gitlab.Collect() function, which can be used to
"easily" collect all items of a specific "list" API (I've implement a
collect function which eagerly collects all items, but something
similar can be implemented for channels to stream).

The only downside of the current implementation so far is that Go
doesn't really support generics / interfaces with direct field access as
of now ... effectively, you'll always need methods.
This means that, because the ListOptions field is somewhat
inconsistently (sometimes as subfield, sometimes it's promoted)
implemented over this package every gitlab.List*Options struct needs
it's own GetPage() int and SetPage(int) implementation ...

One improvement for this could be that we split out the ListOptions
from the API endpoint specific options and make it an additional
argument to the list methods - so that Collect() could profit from
this and GetPage() int and SetPage(int) would only need to be
implemented once.

Another one would be to just use gitlab.ListOptions for endpoints
which don't have additional parameters instead of doing this type ListInstanceVariablesOptions ListOptions.

However, I think best would be to separate the generic ListOptions to
a separate argument in the long term ...

@svanharmelen what do you think?

[2785]

A while back I actually wrote a flavor of this, driven by generics & gammazero's deque that vaguely works like this

func NewProjectMRIterator(ctx context.Context, pid string, opts gitlab.ListProjectMergeRequestsOptions) *ListItemIterator[*gitlab.MergeRequest] {
	return NewListItemIterator(ctx, func(ctx context.Context, page int) (results []*gitlab.MergeRequest, nextPage int, err error) {
		glClient, err := GetGlobalGitlabClient()
		if err != nil {
			return nil, 0, err
		}

		opts.Page = page

		mrs, resp, err := glClient.MergeRequests.ListProjectMergeRequests(pid, &opts, gitlab.WithContext(ctx))
		if err != nil {
			return nil, 0, err
		}

		return mrs, resp.NextPage, nil
	})
}

(if you ignore the global client business)

It's basically

type Iterator[T any] interface {
	Next() (T, error)
}

then you can have your usual DrainIterator[T any](i Iterator[T]) ([]T, error) style utils all you want.

but backed by a queue & goes to fetch a new batch with a user supplied function, when the queue is empty.

Can put a PR up if yall fancy.