Space: Membership Attributes

[luke-]

An Admin should be able to set following additional optional attributes:

• space_membership.show_at_dashboard
• space_membership.can_cancel_membership

https://marketplace.humhub.com/module/rest/docs/html/space.html#tag/Membership/paths/~1space~1%7Bid%7D~1membership~1%7BuserId%7D/post

[gevorgmansuryan]

https://github.com/humhub/rest/pull/116 https://github.com/humhub/humhub/pull/6240