[Workspace] Support workspace level ui settings

[ruanyl]

Is your feature request related to a problem? Please describe.

Workspace is a newly introduced feature to OpenSearch-Dashboards which allows user to manage saved objects isolated by workspaces. There is the need to have some ui settings be customized for an individual workspace instead of having that configured globally( via Advance settings).

Also, ui settings such as default index pattern is going to be problematic under workspace if there is only one global value, because index patterns are workspace-isolated objects, sharing a single global value will not work for all workspaces. This issue is created initially to resolve this problem.

This is part of the story of System settings, Workspace settings and User settings, more details could be found here #4298. For now, we will start with an implementation which has the minimum impact to the rest of the system and keep it flexible for future changes. There are a few open questions are listed below.

Describe the solution you'd like

When calling uiSettings.set within a workspace, the value will be set to the current workspace(as a property of the workspace object) instead of the global config object. And calling uiSettings.get will return the value of the current workspace if the value is set, otherwise, the global value is returned.

Open questions

• How to handle workspace ui settings export and import?
• Is workspace settings migration necessary?

Describe alternatives you've considered

Not yet

Additional context

#4298

[BionIT]

@ruanyl Looks like the PR has been merged as part of 2.14, is this issue done?

[ruanyl]

@BionIT Thanks, closing now