Clarify JSON marshalling of `Decimal` values

## About

How to marshal values of Python's [Decimal](https://docs.python.org/3/library/decimal.html) type.

## Status quo

Currently, the library is conveying Decimal values as strings.

https://github.com/crate/crate-python/blob/7c7a88521d48a9d5225dbd3842f3c4c07fafd4d8/src/crate/client/http.py#L105-L106

## See also

Others apparently need serialization to float.

```python
if isinstance(obj, Decimal):
return float(obj)
```
-- https://github.com/crate-workbench/meltano-target-cratedb/blob/148a2d5ec7131658c1a74aed7d91f23befc81341/target_cratedb/sqlalchemy/patch.py#L65-L66

## References

- https://github.com/crate-workbench/meltano-target-cratedb/issues/52