Better support for custom fields

[andygrunwald]

Context

The current version has many issues with Custom Fields of Jira. See https://github.com/andygrunwald/go-jira/search?q=custom&type=issues

To handle custom fields, the library is using github.com/trivago/tgo

Goal is this task

• Ease out the handling of Jira Custom fields

This request comes from #200.
An own ticket was created to keep the development contained and focus the discussion.

[andygrunwald]

From #200

https://developer.atlassian.com/server/jira/platform/jira-rest-api-examples/
https://docs.atlassian.com/software/jira/docs/api/REST/8.0.2/#api/2/customFields-getCustomFields

Custom fields are defined here. It would be useful to have definitions for each customField type, and additional points for being able to generate code for a project based on the custom fields defined for that project. I have yet to encounter a Jira Library that makes working with custom fields an enjoyable experience.

[rhysjtevans]

This would be awesome