Add signaturesAggr & signatures

[Marketen]

In a mongodb with 3 signatures (where 2 of them have the same pubkey & network):

endpoint /signaturesAggr returns all signatures of the same validator (defined by pubkey and network) in an array:

request: POST to mongodb with json body:

{
    "pubkey": "0xb48c495c19082d892f38227bced89f7199f4e9b642bf94c7f2f1ccf29c0e6a6f54d653002513aa7cdb56c88368797ec",
    "network": "stader"
}

reponse:

[
    {
        "label": "example_label",
        "network": "stader",
        "pubkey": "0xb48c495c19082d892f38227bced89f7199f4e9b642bf94c7f2f1ccf29c0e6a6f54d653002513aa7cdb56c88368797ec",
        "signatures": [
            {
                "platform": "dappnode",
                "signature": "0xa8b00e7746a523346c5165dfa80ffafe52317c6fe6cdcfabd41886f9c8209b829266c5000597142b58dddbcc9c23cfd81315180acf18bb000db50d08293bc539e06a7c751d3d9dec89fb441b3ba6aefdeeff9cfed72fb41171173f22e2993e74",
                "timestamp": "185921877"
            },
            {
                "platform": "dappnode",
                "signature": "0xa8b00e7746a523346c5165dfa80ffafe52317c6fe6cdcfabd41886f9c8209b829266c5000597142b58dddbcc9c23cfd81315180acf18bb000db50d08293bc539e06a7c751d3d9dec89fb441b3ba6aefdeeff9cfed72fb41171173f22e2993e74",
                "timestamp": "185921877"
            }
        ]
    }
]