[Feature] Support adding images for spaces

[tehbooom]

Is your feature request related to a problem? Please describe.
Currently the provider can only change the color of spaces. Would like an option to upload images for spaces.

The kibana spaces api supports adding an image with imageUrl

Describe the resource you would like to have implemented.
Add an optional field for image url like we do with color in the elasticstack_kibana_space resource.

Describe the solution you'd like
As of now the go-kibana-rest package does not support an option for the imageUrl. Once the package has this option or we find another way to add that as an option, we would then be able to add an optional field for imageUrl. If imageUrl is specified color cannot be specified.

Describe alternatives you've considered
go-kibana-rest does not seem to be up to date or current with kibana features. I suggest forking this repo, commiting to this repo, or using net/http package to interact with the kibana API since Elastic does not have an SDK for kibana.

[tobio]

If you were looking to add this yourself, we're already using a local fork of go-kibana-rest since it appears to be abandoned by the maintainer.