extsvc/bitbucketserver: Implement User listing in Client

Created by: tsenart

This commit implements user listing in the bitbucketserver.Client which is needed as part of #1108.

API documentation: https://docs.atlassian.com/bitbucket-server/rest/6.1.2/bitbucket-rest.html#idp363

Test plan: go test

Created by: codecov[bot]

Codecov Report

Merging #4352 into master will increase coverage by 0.02%. The diff coverage is 65.78%.

Impacted Files	Coverage Δ
pkg/extsvc/bitbucketserver/client.go	50.31% <65.78%> (ø)

Created by: tsenart

@mrnugget, @keegancsmith: Please take another look. Addressed most of your feedback.

Created by: mrnugget

@mrnugget, @keegancsmith: Please take another look. Addressed most of your feedback.

LGTM!

Created by: keegancsmith

LGTM