Add ability to disable unused keyword warning

[mtihlenfield]

Is your feature request related to a problem? Please describe.
In cases were I'm writing keyword libraries, the keywords that I'm writing are not necessarily used in the current workspace. This means that the LSP adds the following warning to most (if not all) of those keywords:

The keyword: '<keyword name>' is not used in the workspace.

Describe the solution you'd like
It would be nice to have a way to disable this warning at a workspace or server level. Since this error comes from the LSP and not robocop, I can't find a way to do this.

Describe alternatives you've considered
I have not been able to find an alternative way of doing this in the documentation, but I could just be missing it.