Drop support of (enrichment) option

[yuri-sergiichuk]

It was decided that (enrichment) option should be removed. The reason behind such a decision is a potential ability of a developer to define enrichments from different places at the same time: e.g. from an event that should be enriched and from the enrichment itself.

message Enrichment {
    string user_name = 1 [(by) = "user_id];
}

message SecondEnrichment {
    option (enrichment_for) = "Event";

    string user_address = 1 [(by) = "user_id];
}

message Event {
    option (enrichment) = "Enrichment";
    
    int32 user_id = 1;
}

The enrichment is smth that adds some additional information on top of a message and it is not a responsibility of the message to define how this information is linked to the message itself.

Enrichments should be defined only with (enrichment_for) option.