Create an enum for endpoint_type field in ClaimedExecution struct

[ShubhranshuSanjeev]

#[derive(FromRow)]
pub struct ClaimedExecution {
    pub execution_id: String,
    pub job_id: String,
    pub endpoint: String,
    pub endpoint_type: String,
    pub input: Option<serde_json::Value>,
    pub attempt_count: i64,
    pub max_attempts: i64,
}