Add Icon schema

[tomasciccola]

Icon schema would store information for retrieving an Icon blob, which will be stored directly in an Hypercore. The schema is roughly:

message Icon {
  message IconVariant {
    size: 'small' | 'medium' | 'large';
    number pixelDensity: 1 | 2 | 3;
    string blobVersionId;
    string mimeType;
 }
 Common common;
 string: name;
 repeated IconVariant variant:  
 
}