feat: Add sprint goal

[Avarei]

Description

Adds the optional Goal field to the Sprint Struct

Example:

// Example code
sprints, _, _ := jiraClient.Board.GetAllSprints(boardID)
fmt.Println(sprints[0].Goal)

Checklist

• Unit or Integration tests added
  • Good Path
  • Error Path
• Commits follow conventions described here:
  • Conventional Commits 1.0.0
  • The seven rules of a great Git commit message
• Commits are squashed such that
  • There is 1 commit per isolated change
• I've not made extraneous commits/changes that are unrelated to my change.

[andygrunwald]

Hey,

I am very sorry that this pull request has been open for a long time with no final feedback or merge/decline. We work on this project in our spare time, and sometimes, other priorities take over. This is the typical open source dilemma.

However, there is news: We are kicking off v2 of this library 🚀

To provide visibility, we created the Road to v2 Milestone and calling for your feedback in #489

The development will take some time; however, I hope you can benefit from the changes.
If you seek priority development for your pull request + you like to sponsor it, please contact me.

What does this mean for my pull request?

We will work on this pull request indirectly.
We might merge it during the development or pull parts of it into the new version.
This means that during the development phase, we aim to tackle it.
Maybe in a different way like it is currently handled.
Please understand that this will take a while because we are running this in our spare time.

Final words

Thanks for using and contributing to this library.
If there is anything else you would like to tell us, let us know!

[andygrunwald]

Hey @Avarei,

thanks again for this Pull Request.
Since opening this PR, I made several changes to this library. Those have been causing a conflict with your PR.
I was not able to re-use your commit one by one, but I re-used code of this PR and created + merged #588

I attributed the commit to you.
The sprint goal is now part of the struct.

Thanks again.