lspclient: Allow to use experimental LSP server features

some LSP servers (line gopls) indicates many capabilities as experimental (including semtok and inlay hints), but they are stable enough for years and used in vscode and so on purposed patch adds possibility to enable them per lsp server, so adding to lsp config something like

        "go": {
            "command": ["gopls"],
            "commandDebug": ["gopls", "serve", "-rpc.trace"],
            "path": ["%{ENV:GOPATH}/bin","%{ENV:HOME}/go/bin", "%{ENV:USERPROFILE}/go/bin"],
            "rootIndicationFileNames": ["go.mod"],
            "url": "golang.org/x/tools/gopls",
            "highlightingModeRegex": "^Go$",
            "allowExperimental": true,
            "settings": {
               "gopls": {
                 "semanticTokens": true,
                 "hints": {
                    "assignVariableTypes": true,
                    "parameterNames": true,
                    "rangeVariableTypes": true
                  }
               }
            }
        },

will allow to use that features in kate

screenshot with experimental enabled: