Added issue link

[Fank]

What type of PR is this?

• feature

What this PR does / why we need it:

It adds the missing issue link

Special notes for your reviewer:

Additional documentation e.g., usage docs, etc.:

[andygrunwald]

Thanks for this Pull Request @Fank.
This is looking pretty good already!

A few things, that came into my mind:

Own Service

I am not 100% sure if we should run our own service here or consider making this part of the IssueService.
IssueLinks are pretty coupled to Issues.
And the IssueLinkService would be pretty small (only 3 methods).
I don't have a correct answer for me right now, just a thought.

Existing methods

In issue.go, we have already a few IssueLink API methods:

• go-jira/cloud/issue.go

  Line 735 in aea7aec

  func (s *IssueService) DeleteLink(ctx context.Context, linkID string) (*Response, error) {

• go-jira/cloud/issue.go

  Line 1022 in aea7aec

  func (s *IssueService) AddLink(ctx context.Context, issueLink *IssueLink) (*Response, error) {

They are (still) a wild mix between On-Premise and Cloud, but I would consider removing those already.

Moving forward

Let me know your thoughts.
I can make the changes myself, while merging this into in the next couple of time/days.