Custom field support

[Ruin0x11]

Adds support for custom fields.

• Main behavior is controlled via org-jira-issue-custom-fields. Can set field type, display name and location (placed under a headline or property).
• By default, the field name is the readable name provided by the Jira API, sanitized for org property use. It can be overridden by the above alist.
• The built-in org-jira issue slots like description and assignee can now be overridden by custom fields using org-jira-issue-field-overrides-alist. This is in case the organization uses a custom field for the description/etc. of the issue instead of the default. Internally the field ID is converted to and from the slot name recognized by org-jira when interacting with the Jira API.

[Nazar65]

This is a really good one, it can resolve #355

[JordanAnthonyKing]

Are there any issues with this? Looking forward to seeing it merged. Does it also support updating/progressing issues with custom fields?

[seanhagen]

I'd really like to see this get merged in, my team uses additional fields for things like Success Criteria that I'd like to see in the issue when it gets downloaded.

[ahungry]

@Nazar65 @JordanAnthonyKing @seanhagen can any of you that are interested in this please test it out and report if you find it behaves as expected?

You can clone this repo (or rather @Ruin0x11 copy of it), checkout their branch, and (add-to-list 'load-path "~/path/to/the/clone/location")

This is a large PR and I don't want to merge without additional testers/test feedback.

Please confirm no change in behavior from pre-change, for those that do not need these settings.

[LeeRuns]

@Ruin0x11 so can i use this to get custom fields added to the create issue call? I have required fields for our jira instance and want to put those as part of the normal build