Align Casing in Gas Station REST API Fields

[wulfraem]

Currently, the field names are not 100% consistent, e.g.

• gas station's /v1/reserve_gas and /v1/execute_tx use snake casing, e.g. reservation_id
• hook's endpoint uses camel casing, e.g. reservationId

This makes it hard, to re-use the same request types twice for both systems, as originally intended and, makes the fields more confusing.

Also, enum values for the hook are currently (de-)serialized as camel case, e.g. allow/deny/noDecision, which makes (de-)serializarion a bit harder for using IOTA SDKs ExecuteTransactionRequestType type, which itself is serialized Pascal case, so an adapter for this has to be written. So, re-think, how cases for these should be handled.