[Feature Request] Convert json/yaml to pkl object/class

**Problem statement:**
The adoption of jsons/yamls is pretty much spread across the community/orgs. `pkl` goal's to provide **configuration-as-code language with rich validation and tooling**.
For the cases when `pkl` tool is the right fit this however entails a rather higher ramp up time for people converting the existing configurations -> pkl -> existing configuration format (json/yaml).

**Feature Request:**
A converter that would map the existing configurations into pkl templates + modules.
Aware of the complexity of the existing json/yaml configurations, the first versions of the converter could support the simplest mappings && let the configuration authors to convert more complex definitions manually.

The issue names no files, tests, or entry points. Start by locating the existing JSON/YAML parsing and Pkl module-generation surfaces; done means a converter handles the agreed simplest mappings and has tests defining its supported input and output.

Written by the indexing model from the issue text.