Adjust Bitbucket Cloud issues worker to be resumable

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What does this MR do and why?

Adjust Bitbucket Cloud issues worker to be resumable. This is a continuation from this MR, but for "Issue Worker".

This MR also change the feature flag from wip to beta. Quoting from the doc.

Once the feature is complete, the feature flag type can be changed to the gitlab_com_derisk or beta type depending on how the feature will be presented/documented to customers.

There are 2 more components to be worked on:

• pull_requests_notes
• issues_notes

But both of them need a refactoring to move the API call from "Importer Worker" to "Stage Worker", similar with this issue.

Changelog: performance

MR acceptance checklist

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Screenshots or screen recordings

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Before	After
	Successful imported issues:
	Interruption happened after issue title issue 40:
	Upon resume, retrying issue title issue 40. Then continuing to MR title issue 41:

How to set up and validate locally

1. Setup Bitbucket Cloud following this guide

2. Enable issue tracker by following this guide

3. In rails console enable the feature flag

   Feature.enable(:bitbucket_import_resumable_worker)

4. Patch the code to add interruption:

diff --git a/lib/gitlab/bitbucket_import/parallel_scheduling.rb b/lib/gitlab/bitbucket_import/parallel_scheduling.rb
index c4703c71da9a..7a89b2232a80 100644
--- a/lib/gitlab/bitbucket_import/parallel_scheduling.rb
+++ b/lib/gitlab/bitbucket_import/parallel_scheduling.rb
@@ -42,6 +42,16 @@ def each_object_to_import
         options = collection_options.merge(next_url: page_keyset.current)
 
         client.each_page(collection_method, representation_type, repo, options) do |page|
+          if collection_method == :issues
+            log_info(message: page.inspect)
+            Gitlab::Redis::SharedState.with do |redis|
+              temp_key = 'test-bitbucket-pr'
+              temp_counter = redis.incr(temp_key)
+              redis.expire(temp_key, 5.minutes)
+              raise "purposely interrupt" if temp_counter == 2
+            end
+          end
+
           page.items.each do |object|
             job_waiter.jobs_remaining = Gitlab::Cache::Import::Caching.increment(job_waiter_remaining_cache_key)

5. Visit Bitbucket Cloud import page http://127.0.0.1:3000/import/bitbucket/status
6. Click the Import button.
7. Tail the log file: tail -f log/importer.log

Related to #448895