[re #12346] Adjust mapping logic and behavior based on client specifications, prepared custom_field methods

In this MR, I went throught most of the explanations/requirments from last message frmo Mr. Pícha and worked on resolving them. What I achieved.

• Table project - we can update (at the end of data fetching) the startDate and endDate values - as was mentioned by Mr. Pícha
  • startDate - takes the minimum from the iteration and workunit (the "oldest" date)
  • endDate - takes the maximum from the iteration and workunit (the most "newest" date)
  • to achieve that, I added a couple of new methods into core services and into repositories (I needed to update the project which was not implemented yet - only selecting and creating)
• Removed some TODOs that were there as a placeholders for questions
• The creation of default Unresolved status was removed - by default, we map Unresolved (value null returned from Jira API) into unfinished
• Prepared methods in JiraRestClientService for fetching custom fields
  • These fields will enable us "advanced" data fetching
  • e.g. thanks to them, Jira Users can specify their own Severity (table severity)
  • Right now, only methods are prepared and we will need to ask Mr. Pícha which fields (including Severity) are we gonna save/use

The goal of next MRs will be to implement finally a new features (e.g. extend the current scope of tables, but this was neccessary - we can now call more tables 100% done).

In case of any questions, John Pork (a.k.a. @jpvlck - my biggest fan and CR reviewer),

you can type me on Discord and I will explain more details.