Add top-level config ref

[NivGreenstein]

Is your feature request related to a problem? Please describe.
We at Vector team deploy the same service with 99% the same config for different projects (e.g. Metatile-Queue-Populator/ Retiler). The only difference might be the queue which I'm subscribing to or DB name/ S3 Bucket/ directory in a S3 bucket. Currently, I'll have to create config-per-instance and maintain each one of them (which will result in duplicate config - in the current state of schema and service).
I also expect that I would be able to override properties from the shared config.

Describe the solution you'd like
I would like to be able to top-level $ref a "shared" config in the specific project config's and inherit it's config and override the properties which are project-specific.

Describe alternatives you've considered
Change the schema (again).

Additional context
This will enable us to create ts-server-boilerplate shared config and many more copy-paste config
An example

// shared schema
{                
    "foo": {
        "type": "string"
    },        
    "bar": {
        "type": "string"
    }
}

// shared config
{
    "foo": "avi1",
    "bar": "avi2"
}

// "full" schema
{
    "$ref": "#/ref/to/shared/schema",
    "baz": {
        "type": "string"
    }
}
// => which results to
{
    "foo": {
        "type": "string"
    },
    "bar": {
        "type": "string"
    },
    "baz": {
        "type": "string"
    }
}

// "full" config
{
    "$ref": "#/ref/to/shared/config",
    "baz": "avi3"
}
// => which results to 
{
    "foo": "avi1",
    "bar": "avi2",
    "baz": "avi3"
}

//another valid edge-case option
{
    "$ref": "#/ref/to/shared/config",
    "foo": "avi420"  
    "baz": "avi3"
}
// => which results to 
{
    "foo": "avi420",
    "bar": "avi2",
    "baz": "avi3"
}

Points to consider:
I think we should also have Pick-Omit $ref support in order to, as you probably already guessed, pick a config value/ omit one.

[NivGreenstein]

@CptSchnitz @netanelC @shimoncohen
Please consider :)

[shimoncohen]

@netanelC IMO top-level ref is something useful.
Regarding pick and omit, I think that it depends on the config hierarchy design, which I believe should be built with many small components (meaning there is no real need for pick or omit, but instead using many ref to dedicated configurations).