Add Bitbucket server/Bitbucket Data Center provider for git commit status

[gdasson]

Atlassian Bitbucket product offers multiple hosting options as documented here

For Flux users that use Bitbucket server or Bitbucket data center solution(mostly enterprises), there is no provider to support posting Git Commit Statuses. This PR provides the git commit status functionality for Bitbucket Server/Data Center.

[ Note: The reason that we can't use existing bitbucket provider and the need for a new bitbucketserver provider stems from the fact that the REST API used by Bitbucket Server/Data Center product is different from Bitbucket Cloud REST API. The existing bitbucket provider can only support Bitbucket cloud. ]

This PR fixes issue: #507

[gdasson]

Screenshots attached to show how the Git Commit Statuses will look like on the Bitbucket Server UI once this is implemented.

[makkes]

Left a couple of comments. My main concern is the pulling-in of that library.

[gdasson]

@makkes : Thanks for the review and feedback. I have reimplemented the http calls using resty/http library.

[makkes]

@makkes : Thanks for the review and feedback. I have reimplemented the http calls using resty/http library.

I really don't see the need to pull in any additional library just for a couple of very simple HTTP calls that can just as well be made using net/http.

[gdasson]

@makkes I have reimplemented the HTTP calls using native net/http library/package. Please let me know for any other feedback. Thx.

[makkes]

Looks much better now. I added a couple of additional comments.

[darkowlzz]

The code looks great to me.
Left a few minor suggestions. I think we'll ready to merge after that.

Also, please rebase the commits and squash them to a few relevant commits if possible.

[makkes]

Just some formatting suggestions. Rest lgtm module what darkowlzz asked for.

[makkes]

Not sure what happened here but now there's 2 commits and also changes to .github/workflows/ which shouldn't be here. Please rebase on top of main and squash your commits into one.

[gdasson]

Sorry I had messed up my branch a li'l bit. I have reduced the commits by squashing. Does this look fine now? @makkes @darkowlzz

[makkes]

You still have 3 commits in this PR, two of which are basically the same, plus a merge commit. Please remove the merge commit (rebase instead of merge) and make sure this PR shows a single commit in the "Commits" tab.

[gdasson]

@makkes : Sorry, I was being a bit lazy there 😆. Rebased and squashed down to 1 commit now. Please review. Thanks.

[makkes]

Just one last bit, then we're done here. Great job!

[makkes]

Suggested change

	| [BitbucketServer](#bitbucket-serverdata-center) | `bitbucketserver` |
	| [Bitbucket Server/Data Center](#bitbucket-serverdata-center) | `bitbucketserver` |

[makkes]

Oops, selected the wrong item in the previous review.

[stefanprodan]

Thanks @gdasson 🏅