display feature flags status

[antonkovalenko]

1. Add new tab for cluster - Config
2. Display feature flags as on the sketch

[StekPerepolnen]

there is commit in main
@sareyu new handler invoke example /viewer/feature_flags?features=EnableTempTables,EnableStatistics

{
    "Version": 1,
    "Databases": [
        {
            "Name": "/slice",
            "FeatureFlags": [
                {
                    "Name": "EnableStatistics",
                    "Enabled": true,
                    "IsDefault": true
                },
                {
                    "Name": "EnableTempTables"
                }
            ]
        },
        {
            "Name": "/slice/db",
            "FeatureFlags": [
                {
                    "Name": "EnableStatistics",
                    "Enabled": true,
                    "IsDefault": true
                },
                {
                    "Name": "EnableTempTables",
                    "Enabled": true
                }
            ]
        }
    ]
}