Add the possibility to set level of validation on events

### Description

Today eiffelib validates all events against the Eiffel JSON schemas and, as such, the eiffellib will never allow any events that do not follow the Eiffel specification.
This can cause some annoyances, especially when updating Eiffel specification versions.

### Motivation

The ability to disable validation would end the problems with eiffellib rejecting events on subscribe and publish.

### Exemplification

When a very minor update is done to the Eiffel specification, the eiffellib would not reject the events.

### Benefits

Easier to just let a program execute and hope that it does not break when new updates to the Eiffel specification come.

### Possible Drawbacks

It would allow anyone to just publish and subscribe to any event they want through the eiffellib, even if it's not Eiffel. There is a risk that an internal specification, that does not follow Eiffel, could be created due to having this ability to ignore validation.

ファイル、テスト、エントリーポイントは指定されていません。まず、eiffel-pythonlib が publish および subscribe の際にイベントを検証する場所を特定し、次に、Eiffel 仕様の軽微な更新を許容するというリクエストに対して、選択可能な検証レベルがどのように動作すべきかを判断します。呼び出し側が意図しない拒否なしに検証レベルを選択でき、Eiffel ではないイベントを受け入れるという明記されたリスクに対処できれば完了です。

索引モデルが issue の本文から書いたものです。