[ESS Billing]: ecu rate field

### Integration Name

Elasticsearch Service Billing [ess_billing]

### Dataset Name

ess.billing

### Integration Version

1.4.1

### Agent Version

8.17.4

### OS Version and Architecture

n/a

### User Goal

Translate the ECU to currency value
[https://github.com/elastic/elasticsearch-chargeback/issues/12](https://github.com/elastic/elasticsearch-chargeback/issues/12)

#### Design
From a user perspective the ECU rate should be configurable under the advanced settings:

- This can be updated when rate's change (e.g when the global exchange rate is being used).
- Should be stored in events for hystorical reference.

The events would get expanded with 2 fields, `ecu_rate` and `total_value`:
```json
{
...
"ess": {
"billing": {
...
"total_ecu": 100,
"ecu_rate": 1.8,
"total_value": 180
...
}
}
...
}
```

### Existing Features

Only ECU available. Requires manual configuration of a pipeline.

### What did you see?

n/a

### Anything else?

_No response_