Add support for custom webhook headers

[RicePatrick]

With 17.1, GitLab now supports the ability to set custom headers on webhooks.

See the API documentation here:

Project:
Add/Edit: https://docs.gitlab.com/ee/api/projects.html#edit-project-hook
Set Header: https://docs.gitlab.com/ee/api/projects.html#set-a-custom-header
Delete Header: https://docs.gitlab.com/ee/api/projects.html#delete-a-custom-header

Group:
Add/Edit: https://docs.gitlab.com/ee/api/groups.html#add-group-hook
Set Header: https://docs.gitlab.com/ee/api/groups.html#set-a-custom-header
Delete Header: https://docs.gitlab.com/ee/api/groups.html#delete-a-custom-header

Note that the value of the header will never come back from the API, you can only ever set it (with these APIs).

Note: Ignore the branch name please; I was intending to submit Group hooks as a separate PR, but just ended up adding it here so they could share the underlying structs in a couple cases :)

[RicePatrick]

@svanharmelen - This PR should be ready for review when you have a moment!

[svanharmelen]

I acknowledge this feels weird *[]*HookCustomHeader, but it allows you to make a difference between an empty slice and no value set at all which in some API is needed. So for consistence sake I want to use this pattern in all option structs.

[RicePatrick]

@svanharmelen - No problem at all, I completely understand! All changes are made 😄

[svanharmelen]

Thanks @PatrickRice-KSC 👍🏻