Remove nested config member in /pipelines/{pipeline_id} API

[imor]

The POST /pipelines and POST /pipelines/{pipeline_id} API endpoints have a body that looks like {..., config: { config: ...}. This nested config member should be flattened.

The nested structure is caused by the following structs:

#[derive(Debug, serde::Serialize, serde::Deserialize, PartialEq, Eq)]
pub struct BatchConfig {
    /// maximum batch size in number of events
    pub max_size: usize,

    /// maximum duration, in seconds, to wait for a batch to fill
    pub max_fill_secs: u64,
}

#[derive(Debug, serde::Serialize, serde::Deserialize, PartialEq, Eq)]
pub struct PipelineConfig {
    pub config: BatchConfig,
}