Add signature array processing

[Marketen]

Adds support for /newSignature endpoint to accept an array of signatures like:

[
  {
    "payload": "eyJwbGF0Zm9ybSI6ICJkYXBwbm9kZSIsICJ0aW1lc3RhbXAiOiAiMTg1OTIxODc3IiwgInB1YmtleSI6ICIweGI0OGM0OTVjMTkwODJkODkyZjM4MjI3YmNlZDg5ZjcxOTlmNGU5YjY0MmJmOTRjN2YyZjFjY2YyOWMwZTZhNmY1NGQ2NTMwMDI1MTNhYTdjZGI1NmM4ODM2ODc5N2VjIn0=",
    "signature": "0xa8b00e7746a523346c5165dfa80ffafe52317c6fe6cdcfabd41886f9c8209b829266c5000597142b58dddbcc9c23cfd81315180acf18bb000db50d08293bc539e06a7c751d3d9dec89fb441b3ba6aefdeeff9cfed72fb41171173f22e2993e74",
    "network": "stader1",
    "label": "example_label1"
  },
  {
    "payload": "eyJwbGF0Zm9ybSI6ICJkYXBwbm9kZSIsICJ0aW1lc3RhbXAiOiAiMTg1OTIxODc3IiwgInB1YmtleSI6ICIweGI0OGM0OTVjMTkwODJkODkyZjM4MjI3YmNlZDg5ZjcxOTlmNGU5YjY0MmJmOTRjN2YyZjFjY2YyOWMwZTZhNmY1NGQ2NTMwMDI1MTNhYTdjZGI1NmM4ODM2ODc5N2VjIn0=",
    "signature": "0xa8b00e7746a523346c5165dfa80ffafe52317c6fe6cdcfabd41886f9c8209b829266c5000597142b58dddbcc9c23cfd81315180acf18bb000db50d08293bc539e06a7c751d3d9dec89fb441b3ba6aefdeeff9cfed72fb41171173f22e2993e74",
    "network": "stader2",
    "label": "example_label2"
  },
  {
    "payload": "eyJwbGF0Zm9ybSI6ICJkYXBwbm9kZSIsICJ0aW1lc3RhbXAiOiAiMTg1OTIxODc3IiwgInB1YmtleSI6ICIweGI0OGM0OTVjMTkwODJkODkyZjM4MjI3YmNlZDg5ZjcxOTlmNGU5YjY0MmJmOTRjN2YyZjFjY2YyOWMwZTZhNmY1NGQ2NTMwMDI1MTNhYTdjZGI1NmM4ODM2ODc5N2VjIn0=",
    "signature": "0xa8b00e7746a523346c5165dfa80ffafe52317c6fe6cdcfabd41886f9c8209b829266c5000597142b58dddbcc9c23cfd81315180acf18bb000db50d08293bc539e06a7c751d3d9dec89fb441b3ba6aefdeeff9cfed72fb41171173f22e2993e74",
    "network": "stader3",
    "label": "example_label3"
  }
]

Processing is done as follows:

1. Request data structure validation --> check that all elements of the slice have the correct fields and values
2. Decode payloads --> ensure decoded BASE64 payload has expected "platform", "timestamp" & "pubkey" fields.
3. Verify onchain validators: Call consensus client with request's valid pubkeys to identify which are from real validators
4. Verify signatures: Verify signatures from real validator pubkeys
5. Add to mongodb: Add valid signed messages to mongodb