Add API Support for Kubernetes integration

What does this MR do?

Add Kubernetes Integration for the following endpoints:

GET :id/clusters # Returns project clusters
GET :id/clusters/:cluster_id # Returns specific cluster
POST :id/clusters/user # Add a cluster to the project. Same as "Add existing Cluster" from UI
PUT :id/clusters/:cluster_id # Update existing cluster
DELETE :id/clusters/:cluster_id # Removes cluster

EE counterpart: gitlab-ee!8994

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/40473

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated -https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24135
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer

Backend - To do

• List all clusters
• Get specific cluster
• Add existing cluster
• Update a cluster
• Destroy a cluster
• Manual QA

Manual QA

Manual QA was done using an EE environment

GET :id/clusters - Returns project clusters

With developer & maintainer access
index

GET :id/clusters/:cluster_id - Returns specific cluster

With developer & maintainer access
GET

POST :id/clusters/user - Add a cluster to the project. Same as "Add existing Cluster" from UI

With valid params	With invalid params
POST_valid	POST_invalid

PUT :id/clusters/:cluster_id - Update existing cluster

With user cluster	With gcp cluster
PUT_USEr	PUT_GCP

DELETE :id/clusters/:cluster_id - Removes cluster

With existing cluster	With unknown cluster
DELETE	DELETE_unknown