Add some way to disable diagnostics from a given server

[Jorengarenar]

Related:

• How to disable completion and diagnostics capabilities of a server? #1573
• Disable capabilities of individual language servers #1505
• Disable diagnostics for one of the langservers #1163
• How to disable formatting for a particular server? #1056

Sometimes diagnostics by some servers aren't very useful. In such cases, it would be helpful to disable diagnostics send from them. Unfortunately, seems like for now vim-lsp ask all servers for diagnostics indiscriminately.