hscim client

[fisx]

I need client functions for https://github.com/wireapp/ldap-scim-bridge. This PR introduces them to the hscim library.

Please read commit-by-commit.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.
• If end-points have been added or changed: the endpoint / config-flag checklist (see Wire-employee only backend wiki page) has been followed.
• If a schema migration has been added, I ran make git-add-cassandra-schema to update the cassandra schema documentation.
• Section Unreleased of CHANGELOG.md contains the following bits of information:
  • A line with the title and number of the PR in one or more suitable sub-sections.
  • If /a: measures to be taken by instance operators.
  • If /a: list of cassandra migrations.
  • If public end-points have been changed or added: does nginz need upgrade?
  • If internal end-points have been added or changed: which services have to be deployed in a specific order?

[mdimjasevic]

I left some comments inlined.

[fisx]

Thanks, good feedback! I think I resolved everything, please unresolve the comments where you disagree.

[fisx]

(should the process be that the reviewer resolves the comments, after the PR author has answered them? no strong opinion.)