common: update the validation of the enriched event with static field lengths

[benjben]

In 93078e2 we've introduced the validation of the enriched event according to atomic schema before emitting.

The implementation is to convert the enriched event to a JSON and to use Iglu client to validate it against the schema. Being based on JSON manipulation, we've observed a significant bump in the use of CPU by the enriching step of the app; ~ x5 in case no enrichments are activated (other steps like reading from/writing to message queue being not impacted).

For this reason, for now we'd like to limit the validation to the validation of each field length (which was the original problem we were trying to solve), like this is done in analytics-sdk.

When this issue will be implemented, we'll see to add the full validation with Iglu client again.